Make input event forwarding work in --site-per-process.

This moves forwardInputEvent to WebRemoteFrame and fixes a few uses of
RemoteFrames.

BUG=346764
TEST=Input events work in out-of-process iframes.

Review URL: https://codereview.chromium.org/642203002

git-svn-id: svn://svn.chromium.org/blink/trunk@183569 bbb929c8-8fbe-4397-9dbb-9b2b20218538