GitLab

Move the Chrome Variations HTTP header helper file into c/b/metrics/variations, and fix up lock behavior a bit.

Specifically:
  (1) Reduces the duration for which the lock is held when appending HTTP headers.
  (2) Adds a debug assertion that the lock is correctly held when calling UpdateVariationIDsHeaderValue().

BUG=none

Review URL: https://chromiumcodereview.appspot.com/11821019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175967 0039d316-1c4b-4281-b951-d872f2087c98