Get download icons from GTK in linux_aura.

Add a GetIconForContentType hook in LinuxUI and implement in libgtk2ui using
the native library interfaces. This saves us from having to implement icon
theme lookup ourselves.

BUG=102211
TEST=downloading a file in linux_aura on stock Ubuntu shows icons.

Review URL: https://codereview.chromium.org/70533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234709 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234709