blink/bindings: Improve ParkableStringManager testing.

|ParkableStringManager::SetRendererBackgrounded()| schedules delayed tasks, but
tests were not waiting for the tasks to run, instead directly calling the
delayed functions.

This adds a test checking for proper task sequencing, and changes other tests to
properly wait for tasks. As a consequence, the ParkableStringManager tests no
longer need to be friend with ParkableStringManager, provided that |Size()|
becomes public.

Also fixes a couple "git cl lint" warnings.

Bug: 877044
Change-Id: I15ea4285596dc744969e0bce63c5687e6fed023c
Reviewed-on: https://chromium-review.googlesource.com/c/1331476
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607518}
4 files changed