Media Galleries API Scanning: Add Scan Permission and basic test.

BUG=161119

Review URL: https://codereview.chromium.org/133463003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247894 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247894