Input tasks are not always routed by the main thread, but the
blink scheduler still needs to know about them because we
want it to enter low latency mode when the user is
interacting with the device.  To facilitate that this patch
adds an didReceiveInputEvent API function.

BUG=391005

Review URL: https://codereview.chromium.org/554693004

git-svn-id: svn://svn.chromium.org/blink/trunk@181830 bbb929c8-8fbe-4397-9dbb-9b2b20218538