Revert of Explicitly call SequenceToken's default constructor.

Revert of Explicitly call SequenceToken's default constructor. (patchset #2 id:20001 of https://codereview.chromium.org/2184353003/ )

Reason for revert:
xcode's clang now supports const default initialization.

Original issue's description:
> Explicitly call SequenceToken's default constructor.
> 
> This CL is to workaround a clang bug occurring when building
> sequence_token_unittest.cc for Chrome iOS.
> 
> BUG=632330
> 
> Committed: https://crrev.com/0240044e435a3708d3f082771943a498a4cf3592
> Cr-Commit-Position: refs/heads/master@{#408515}

TBR=danakj@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=632330

[jif, 2017-02-22 10:18:35]

The CQ bit was checked by jif@chromium.org

[jif, 2017-02-22 10:18:36]

Created Revert of Explicitly call SequenceToken's default constructor.

[commit-bot: I haz the power, 2017-02-22 10:19:09]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-02-22 10:33:28]

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

[commit-bot: I haz the power, 2017-02-22 10:33:29]

Try jobs failed on following builders:
  ios-device-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device-xcode-...)
  ios-simulator-xcode-clang on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator-xco...)

[jif, 2017-02-22 10:37:35]

So apparently xcode's clang still does NOT supports const default
initialization.
Closing CL.