Group plugins in about:plugins and show update link for out-of-date ones.

Re-landing patch by mavrommatis, original review here:
<http://codereview.chromium.org/1991005>

Original description follows:
(1) Group plugins with the same name together. (2) Show a download link for
plugin versions with known security problems in about:plugins.

BUG=3910,47858
TEST=Open "chrome://plugins", see that plugins are grouped, and that any vulnerable plugins are marked red. Try enabling and disabling plugin groups. Also, if the internal PDF plugin is enabled, it should stay enabled after a restart.

Review URL: http://codereview.chromium.org/2811037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51510 0039d316-1c4b-4281-b951-d872f2087c98