Revert of Added webrtc/base/safe_conversions.h as a pseudonym

Revert of Added webrtc/base/safe_conversions.h as a pseudonym (patchset #1 id:20001 of https://codereview.webrtc.org/1774933003/ )

Reason for revert:
Looks like the Chrome iOS build is broken because of these two changes.  So I'm going to have to revert.  Here's the error:

https://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ninja/builds/185624/steps/compile/logs/stdio

FAILED: rm -f arch/libsafe_numerics.arm64.a && ./gyp-mac-tool filter-libtool libtool  -static -o arch/libsafe_numerics.arm64.a 
error: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: no files specified
Usage: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool -static [-] file [...] [-filelist listfile[,dirname]] [-arch_only arch] [-sacLT] [-no_warning_for_no_symbols]
Usage: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool -dynamic [-] file [...] [-filelist listfile[,dirname]] [-arch_only arch] [-o output] [-install_name name] [-compatibility_version #] [-current_version #] [-seg1addr 0x#] [-segs_read_only_addr 0x#] [-segs_read_write_addr 0x#] [-seg_addr_table <filename>] [-seg_addr_table_filename <file_system_path>] [-all_load] [-noall_load]
FAILED: rm -f arch/libsafe_numerics.armv7.a && ./gyp-mac-tool filter-libtool libtool  -static -o arch/libsafe_numerics.armv7.a 
error: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: no files specified
Usage: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool -static [-] file [...] [-filelist listfile[,dirname]] [-arch_only arch] [-sacLT] [-no_warning_for_no_symbols]
Usage: /Applications/Xcode7.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool -dynamic [-] file [...] [-filelist listfile[,dirname]] [-arch_only arch] [-o output] [-install_name name] [-compatibility_version #] [-current_version #] [-seg1addr 0x#] [-segs_read_only_addr 0x#] [-segs_read_write_addr 0x#] [-seg_addr_table <filename>] [-seg_addr_table_filename <file_system_path>] [-all_load] [-noall_load]
ninja: build stopped: subcommand failed.

Original issue's description:
> Added webrtc/base/safe_conversions.h as a pseudonym
> for webrtc/base/numerics/safe_conversions.h.
> 
> This prevents downstream projects from breaking that have not yet been
> updated to use the new file path. As soon as they have this file should
> be removed.
> 
> This is a follow-up to https://codereview.webrtc.org/1753293002/.
> 
> TBR=hta@webrtc.org
> NOPRESUBMIT=True
> NOTRY=True
> BUG=webrtc:5548
> 
> Committed: https://crrev.com/b8f78858619c676955e1098ecc24f2612e55c84f
> Cr-Commit-Position: refs/heads/master@{#11912}

TBR=hta@webrtc.org,hbos@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:5548

[tommi (sloooow) - chröme, 2016-03-12 00:46:45]

Created Revert of Added webrtc/base/safe_conversions.h as a pseudonym

[commit-bot: I haz the power, 2016-03-12 00:46:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1789853003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1789853003/1

[commit-bot: I haz the power, 2016-03-12 00:46:49]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-03-12 00:46:50]

No L-G-T-M from a valid reviewer yet. 
CQ run can only be started by full committers or once the patch has
received an L-G-T-M from a full committer.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
Committers are members of the group "project-webrtc-committers".
Note that this has nothing to do with OWNERS files.