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This CL adds prefetch browsertests for main-resources, and subresources to
ensure that the main-resource implementation doesn't break SplitCache. The
tests all run with the SplitCacheByNetworkIsolationKey feature enabled
to prepare for the implementation of Cross-origin Prefetch + SplitCache.
The scenarios tested are as follow:

 - Cross-origin as=document prefetch reuse as top-level-navigation
    - This tests that the cross-origin resource is cached with a
      NetworkIsolationKey corresponding to its origin. Therefore, it can
      be reused for top-level-navigations
 - Cross-origin as=document prefetch not reused as cross-origin subresource
    - This tests that even though cross-origin as=document prefetches are
      cached under an atypical NetworkIsolationKey, their reuse is
      restricted to top-level-navigations.
 - Cross-origin prefetch subresources are not able to be reused from cache
   in a cross-origin context. (AKA, we didn't break SplitCache).
 - Cross-origin prefetch subresources are able to be reused by the frame
   that fetched them. (AKA, we still didn't break SplitCache).
 - Cross-origin prefetch subresources not reused as top-level navigations
    - As the comment in the code suggests, even though this is safe, a
      resource must be committed to a single cache partition. Therefore, if
      a cross-origin subresource is prefetched, it cannot be used anywhere
      else but in the frame that fetched it.

R=kinuko@chromium.org, yhirano@chromium.org

Bug: 939317
Change-Id: Ie50844463a838c5fabd5a153cd0caf720a59da4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732869
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684319}