I have a CL up for this at: https://chromium-review.googlesource.com/c/chromium/src/+/838626 The CL is still WIP, but I think gives a good overview of the final architecture.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8e8ae37d2e02943cadb4d658777866e8e4a12f60

commit 8e8ae37d2e02943cadb4d658777866e8e4a12f60
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Sun Dec 24 03:56:47 2017

event router: Use mojom.InputTargetClient for targeting.

Introduce a RenderWidgetTargeter. Briefly:

. RenderWidgetTargeter is responsible for finding a target for a
  particular event. Right now, it first asks the router itself to find a
  target. The router returns a target, and a bool flag, indicating
  whether the returned target needs to be asked for the final target
  (i.e. whether InputTargetClient should be used).
. Once HitTestQuery is ready, the same RenderWidgetTargeter can be used,
  but instead of asking the router itself for the target, and bool flag,
  the targeter will ask the HitTestQuery.
. If an event-targeting request is sent to a client using the
  mojom.InputTargetClient API, then subsequent events are put in a queue,
  until the client responds to the request.
. Once a client responds to a targeting request, RenderWidgetTargeter
  attempts to flush the queue, until the queue becomes empty, or another
  targeting request to a client is made using InputTargetClient api.

Bug:  796656 
Change-Id: I55a75f9d69136731af96b2bf292c0f361fa729c4
Reviewed-on: https://chromium-review.googlesource.com/838626
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526154}
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/BUILD.gn
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/frame_host/render_frame_host_impl.cc
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/frame_host/render_frame_host_impl.h
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/DEPS
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_host_impl.cc
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_host_impl.h
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_host_input_event_router.cc
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_host_input_event_router.h
[add] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_targeter.cc
[add] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/renderer_host/render_widget_targeter.h
[modify] https://crrev.com/8e8ae37d2e02943cadb4d658777866e8e4a12f60/content/browser/site_per_process_browsertest.cc

I am changing this bug to be for just mouse events.

Fix for mouse-wheel event is tracked in  issue 799473 .
Fix for touch events is tracked in  issue 799474 .