GitLab

This creates a registry of all FrameNodes and WorkersNodes, and a
wrapper that allows them to be accessed from a common ExecutionContext
type. This will be used in various upcoming per-ExecutionContext
tracking CLs.

BUG=1085129

Change-Id: Ibe6467ee07c9e143c788f6a42dcdd955966a0fd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274543
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785994}