Create App Launcher shortcuts and icons on Linux with --enable-app-list.

The shortcuts are only created if they have not been in the past.

This adds Linux-suitable icons to chrome/app/theme/chromium/linux, which
are PNG versions of the images in app_list.ico. The existing PNG images
are designed exclusively for Mac and are unsuitable on Linux.

Also made --enable-app-list and --reset-app-list-install-state available
on Linux in about://flags.

BUG=299250

Review URL: https://codereview.chromium.org/111853006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245855