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- Do not update scroll if the delta is too small, this matches what
  drags on overview items do.
- Pause occlusion tracker on scroll.
- When scrolling, do not call SetBounds unless the item is leaving or
  entering the view of the grid.

Tested locally with OverviewScrollTest.Basic. Not the greatest gains
when only 12 windows, but scales much better with larger number of
windows.

Presentation time:
Before 12 windows: 55ms
Before 42 windows: 180ms
After 12 windows: 53ms
After 42 windows: 88ms

Also renamed a overview_grid_pre_event_handler ->
overview_grid_event_handler as it handles regular events now.

Test: manual
Bug: 978112, 993615
Change-Id: I28bcbb8ce3291f975df9f1033961fbe176052fc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784349
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693921}