Make LastKnownMousePosition return FloatPoint

This CL changes EventHandler and MouseEventManager LastKnownMousePosition
to return FloatPoint.
EventHandler.LastKnownMousePositionInRootFrame has two caller,
1. PaintLayerScrollableArea will floor the point before use it.
2. AutoscrollController will convert FloatPoint to LayoutPoint and use
LayoutPoint instead.
This is not going to change any behavior as auto scroll use main thread
programatic scroll, which does not suport fractions yet.

Bug: 940506
Change-Id: I8ec776793a6dd05d86a3e01cf5f3f0a93815d553
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1510056
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639551}
11 files changed