Add the "follow" algorithm to AbortSignal

Add a Follow() method to AbortSignal that implements this algorithm:
https://dom.spec.whatwg.org/#abortsignal-follow.

Update the Request constructor and clone() method to use the algorithm.

Add a layout test that chains of AbortSignals don't drop aborts when garbage
collection happens.

Bug: 750599
Change-Id: Id4e0df8b81aae3037e324d7787b5ba604ab9efba
Reviewed-on: https://chromium-review.googlesource.com/948384
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541070}
4 files changed