Display Extension icons in about:inspect

BUG=308467

Review URL: https://codereview.chromium.org/27513013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229352 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229352
Add some more documentation about the itunes media gallery.

NOTRY=TRUE
BUG=NONE

Review URL: https://codereview.chromium.org/27518007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229353 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229353