Pull media gallery checkbox into a common class and improve layout.

We want the gallery checkboxes to fit the available horizontal space, and elide the gallery name in the middle. That's accomplished by doing custom layout. Also improves the spacing on the scan result dialog.

BUG=NONE

Review URL: https://codereview.chromium.org/180573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254194 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254194