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Extensions outside Chrome Web Store (self-hosted ones) don't have
hashes signed by Chrome Web Store (stored in verified_contents.json
file), so historically are not checked.

This commit adds support for extensions to be "should check, but
shouldn't verify": if such an extension has computed_hashes.json file,
we'll use it to check extension's content and report appropriately to
delegate.

Bug: 958794
Change-Id: Id8bce11fa42e5a8099f65d3013cb25f2271f51c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735454
Commit-Queue: Oleg Davydov <burunduk@chromium.org>
Reviewed-by: Nikita Podguzov <nikitapodguzov@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712724}