Media Galleries Scanning: Refine when Add Scan Results dialog shown.

Check for new galleries and user gesture before showing add scan results dialog.

BUG=161119

Review URL: https://codereview.chromium.org/148753003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248081 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248081