API to wait for launcher state change.

This also adds api to activate accelerator, which is used
to change the launcher state.

Bug: 1001728
Test: covered by unittests
Change-Id: I9e545a4ab5a646a5260f59b821e3229e87ad5466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869991
Commit-Queue: Mitsuru Oshima <[email protected]>
Reviewed-by: Steven Bennetts <[email protected]>
Reviewed-by: Jun Mukai <[email protected]>
Cr-Commit-Position: refs/heads/master@{#711556}
15 files changed