Pepper: Remove unused Plugin::manifest_url_.

BUG=239656
R=dmichael@chromium.org

Review URL: https://codereview.chromium.org/143623008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250131 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250131