Fix DCHECK in HandleEventMessageFilterHook firing when right clicking on a windowless Silverlight object like http://www.microsoft.com/silverlight/new-controls/demo/.

The problem was that WebPluginDelegateImpl::PlatformHandleInputEvent was setting a message filter hook but if we didn't enter a modal loop in NPP_HandleEvent it wasn't getting unregistered.

R=ananta@chromium.org

Review URL: https://codereview.chromium.org/70103007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236658 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236658