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This updates the High PM policy to make it discard tabs directly rather
than sending memory pressure signals.

Change-Id: I26c85179b8c03268d84c0c99918239f2344bb6d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304106
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790908}