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This will improve fade in performance by delaying the creation of the
heavy view until needed. It will transfer some delay to the tabbing
animation but I think the tradeoff is worth it as, for example with
7 initially offscreen windows:
  1) The performance drop will be split 7 ways (not exactly but rough
  estimate).
  2) Fade in performance is about 3x worse right now and arguably more
  noticeable.

Ran the test below (modified a bit) for 8 and 16 windows, 20 times each
in a loop. Values are first fade in/average fade in/average container,
machine is krane. Though would need to check the dashboards for the
real truth.

Before 8: 6/15/90
Before 16: 4/5/50
After 8: 18/61/89
After 16: 10/58/50

Test: Tast test ui.WindowCyclePerf
Bug: 1078159
Change-Id: I2e7252089b7686b109cd5b196ba847eac29e97bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2197145
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768907}