Tag the scroll message sent to the OOP PDF plugin with its type.

TBR=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/138903004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246033 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246033