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This patch implements committed interstitials for safebrowsing
for subresources (as a follow-up to crrev.com/c/2003857).

In this patch:
- Make sure to only add safebrowsing navigation throttle for
  main frames when committed interstitials are enabled.
- Implement safe_browsing_subresource_helper and update
  safe_browsing_ui_manager to handle subresources (such as
  js files).
- Add a browsertest for testing safebrowsing interstitials
  for subresources (both with and without committed
  interstitials enabled).

BUG=1044593,1042662

Change-Id: I813e916aa27dcb262fcd39d9b20af0f6d5d41e1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2030485
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737338}