ScriptWrappable: Add method-level docs for methods called from GC

Bug: 970150
Change-Id: Iaba850dc87b16d40c4822ad04e95883c0469851b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649157
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667062}