AbortSignal: Model dependent signals as Member instead of root

The root in AbortSignal was creating the following cycle with unified
heap:
  Request -> AbortSignal -> AbortSignal (root) -> ExecutionContext -> Request

Bug: 928781,843903
Change-Id: Iefad41f37fe3ccb3960a7c3d9e1b963730aab7ec
Reviewed-on: https://chromium-review.googlesource.com/c/1477241
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633714}
2 files changed