Drop unhandled ResourceHostMsg's in the ResourceDispatcherHostImpl

Certain ResourceHostMsg's may go unhandled in ResourceDispatcherHostImpl,
depending on whether an appropriate ResourceMessageDelegate exists.  If it goes
unhandled, it is forwarded to all other filters and from there to the
RenderProcessHostImpl.  All of that is needless work as the message will simply
be dropped, so drop the message instead in ResourceDispatcherHostImpl.

BUG=343343

Review URL: https://codereview.chromium.org/164333016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253692 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253692