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This patch adds code that reacts to memory state change signals to
purge unlocked discardable memory when we're throttled or suspended.

R=ericrk@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2364683004
Cr-Commit-Position: refs/heads/master@{#420750}