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The chrome-extension-resource scheme was added in
https://chromiumcodereview.appspot.com/9909019/ but was never used. The code for
it was eventually removed in https://codereview.chromium.org/2574763003.

This CL removes any remaining references to it. Behavior changes:
- Any extensions using the scheme in their CSP will get an install warning as
  the token will be recognised as an unsafe token. Note we don't show install
  warnings for packaged extensions.
- The default CSP for apps and extensions now excludes the scheme. However this
  should not be an issue since the scheme isn't a valid scheme and can't be
  used.

BUG=674272

Change-Id: I4494e901d147627adebc62c59ce0e021876786a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719333Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686197}