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* Adds support for isolated CastRunner instances, which run without
  networking capabilities and only have access to resources served
  by local ContentDirectoryProviders.
* Defers web.Context creation to WebComponent creation time, so that
  CastRunner can defer shared Context creation until it's actually
  needed.

Bug: 1004505
Change-Id: I4af9ca7c04b9c86e93c30b2d0aaa7f39ee3f8d5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808159
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705634}