Added a RAF wait, support for devicePixelRatios other than 1, screenshot archiving, and test expectations support

BUG=143317
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/75133005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236021 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236021