Only permit plug-in loads in the browser if the plug-in isn't blocked or the
user has authorized it with a browser-mediated interaction.
(Reland https://codereview.chromium.org/12086077 with Android test tweak)
BUG=172573
R=jam@chromium.org
Review URL: https://codereview.chromium.org/12092107
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@180159 0039d316-1c4b-4281-b951-d872f2087c98