Media Galleries: Initial media scanner implementation. (try 2)

BUG=161119

Review URL: https://codereview.chromium.org/144853007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248074 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248074
Fix for flaky MergeSession test series.

TEST=OAuth2Test.*
BUG=none
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/129863005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248075 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248075