Prevent OOB read from plugin calls returning strings with embedded NUL characters. Duplicates this Pepper fix: http://src.chromium.org/viewvc/chrome?view=rev&revision=82172

      BUG=77493

Review URL: http://codereview.chromium.org/7099006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88758 0039d316-1c4b-4281-b951-d872f2087c98