WindowService: renames EventDispatch to EventProcessor

Using 'dispatch' is mildly confusing given the class doesn't really
handle dispatch.

BUG=none
TEST=none

Change-Id: I20f858d924d0a68efb2e449fd0ab3140ef5ffa54
Reviewed-on: https://chromium-review.googlesource.com/956496


Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542184}