Begin writing reference images for failed diffs into the generated dir.

BUG=310917

Review URL: https://codereview.chromium.org/42523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230956 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230956
settings: Add async system salt retrieval logic in DeviceOAuth2TokenServiceFactory

Along the way, remove GetCachedSystemSalt() from SystemSaltGetter.

BUG=309959, 306547
TEST=none
R=hashimoto@chromium.org, pastarmovj@chromium.org

Review URL: https://codereview.chromium.org/39443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230957 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230957