autoroller: Add TBR= field and always update the checkout

autoroller: Add TBR= field and always update the checkout

Add a TBR= field to enable automated rolling. In some cases, add a
committer to the field, for other rolls: leave it empty.

Always run 'git pull', even if --dry-run is specified, as
it is often used to just generate an updated commit message to
update an existing CL with when updating it to match fixes in Chromium.

BUG=webrtc:4688
R=phoglund@webrtc.org

Committed: https://crrev.com/6c2ba7d13c1a0f733fa993f3e5145ea795d356ce
Cr-Commit-Position: refs/heads/master@{#10140}

[kjellander_webrtc, 2015-10-01 15:56:36]

Patchset #1 (id:1) has been deleted

[kjellander_webrtc, 2015-10-01 15:57:38]

kjellander@webrtc.org changed reviewers:
+ phoglund@webrtc.org

[kjellander_webrtc, 2015-10-01 15:57:38]

Having TBR= is needed if we're going to automate this.

I hacked in some authors that usually wants to get notified when libvpx/clang
changes (since there's no way to get notification about that using WATCHLISTS)
due to how our checkout is setup.

[phoglund, 2015-10-02 06:50:02]

lgtm

[kjellander_webrtc, 2015-10-02 07:03:32]

Committed patchset #1 (id:20001) manually as
6c2ba7d13c1a0f733fa993f3e5145ea795d356ce (presubmit successful).

[commit-bot: I haz the power, 2015-10-02 07:03:34]

Patchset 1 (id:??) landed as
https://crrev.com/6c2ba7d13c1a0f733fa993f3e5145ea795d356ce
Cr-Commit-Position: refs/heads/master@{#10140}