Defend against content confusions attacks by Flash.

      If the HTTP header for the intial Flash plug-in file indicates no sniffing,
then insist that that Content-Type is either blank, or fully correct
before loading the plug-in.

BUG=75070
Review URL: http://codereview.chromium.org/7373009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@92717 0039d316-1c4b-4281-b951-d872f2087c98