Do not create app_list item's layer until it's necessary

This improves the animation smoothness for expanding to fullscreen
from peeking, from ~20 to ~65 for oneshot, and ~80 to ~90 for
reuse.

Still sometimes (1 in 5 attempts) ~20 animation smoothness can be
observed.

This also improves drag-to-reveal performance; LauncherDragTest.Open
is improved by ~10msecs (~80 to ~70).

Bug: 978179, 961467
Test: interactive_ui_tests
Change-Id: Ia0550cada6cedb8525dee8472e89de9c1d5e246f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1673575
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672145}
5 files changed