This changes gin::Wrappable to track its cleanup
state in order to prevent async operations from
recreating a wrapper between first and second weak
callbacks.

GetWrapper is changed to return a MaybeLocal,
and callers are updated accordingly; checking
in some cases and failing gracefully in others.

Mojo JS's WaitingCallback is changed to silently
ignore handle notifications if its wrapper is
no longer alive.

BUG=707689

Change-Id: I3fc11a24209f0ef35bfa18556f5734c4b18ae229
Reviewed-on: https://chromium-review.googlesource.com/475077


Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#464134}