Move content/renderer input handling for web input events to ui.

The InputHandlerProxy in content/renderer implements the logic for
sending the intercepted web input events to the compositor. Move this
logic to ui to be used by mus/blimp.

BUG=550531

Review URL: https://codereview.chromium.org/1415953004

Cr-Commit-Position: refs/heads/master@{#361207}