Add plumbing to allow input events to be routed to embedder if swapped out.

This change is needed for the initial version of out-of-process iframes to route events to other processes. The corresponding Chromium side patch is https://codereview.chromium.org/130433016/.

BUG=305811,339659

Review URL: https://codereview.chromium.org/138003011

git-svn-id: svn://svn.chromium.org/blink/trunk@166340 bbb929c8-8fbe-4397-9dbb-9b2b20218538