Add test code for readback comparision with more image pattens.

BUG=323150

Review URL: https://codereview.chromium.org/151203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251307 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251307
Repurpose NotificationBitmapFetcher to BitmapFetcher

There was nothing specific about this class, so I'm generalizing it and moving it upwards in the tree for more use cases (e.g. app banners).

Code that will use it is not ready for review, but is located at https://chromiumcodereview.appspot.com/156343002/

BUG=341556

Review URL: https://codereview.chromium.org/155273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251308 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251308