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This removes MC autoupgrades for blockable content (including
websockets) and adds a parameter to disable them for images. This
matches the behavior planned for launch.

Bug: 1025274, 1027686
Change-Id: Iaef5800c09368a547f6301c88208c535b60f9bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1928239Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Auto-Submit: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718332}