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As the mouse cursor moves across OOPIF boundaries, elements in
other processes that have certain mouse event handlers might require
notification in order to fire those handlers. An example is MouseLeave,
which is not currently fired when the MouseMove is directed to a
parent frame.

This CL tracks the last RenderWidgetHostView to receive a mouse
event and sends appropriate events to other renderers when that changes.

BUG=632035

Review-Url: https://codereview.chromium.org/2229463004
Cr-Commit-Position: refs/heads/master@{#418629}