v8binding: Introduce WorldSafeV8Reference class

WorldSafeV8Reference class provides safe access across worlds
plus wrapper-tracing.

The usage is demonstrated at CustomEvent.detail.

Change-Id: I6c640d916063ac87604bd63f25524f8a4f8dd817
Bug: 501866, 755520, 803478
Reviewed-on: https://chromium-review.googlesource.com/c/1358611
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615844}
5 files changed