GitLab

Moves one more method from PersistentPrefStore to WritablePrefStore. This allows most "writing" operations to be possible without the more complex PersistentPrefStore API.

BUG=None

Review URL: https://codereview.chromium.org/389333003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282967 0039d316-1c4b-4281-b951-d872f2087c98