Update the event filter order about partial screenshot.
PartialScreenshotController prepends itself to the event filters.
This means that mouse events are handled prior to any other
event filters including mouse cursor event filter which controls
the bigger cursor or the cursor in the software mirroring display.
So we need to be careful of the event filter order, mouse cursor
event filter should be processed first, and then partial screenshot
may handle the event. This means partial screenshot event handler
has to be created in Shell and added to event filters prior to
mouse cursor event filter (because they are prepended).
BUG=459214
R=oshima@chromium.org
TEST=The new test case covers
Review URL: https://codereview.chromium.org/
942083006
Cr-Commit-Position: refs/heads/master@{#317904}