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Currently a *compromised* web renderer can request any extension
resource from an extension that has web accessible resources (WAR). This
was necessary earlier since:

  - extension iframes didn't always live in the extension process when
  embedded in a website.

  - We needed to allow extension subresource requests from such iframes.
  These subresources didn't need to be web accessible.

  - This meant that the browser needed to allow cross renderer requests
  to such resources.

  - Hence as a compromise, at the browser level we allowlisted any cross
  renderer subresource request to an extension with WAR.

However, now extension iframes should always have their own process.
Hence the browser side check can be made more strict to disallow cross
renderer requests to non web accessible resources.

Note that we already block access to non web accessible extension
resources at the renderer level.

BUG=1093570, 179127, 173688.


Change-Id: I20523db2d1629f3714d3c84a27e0904ed4f1b27a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2244433
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Auto-Submit: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779744}