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This CL lets us schedule a purge of unlocked discardable memory from the
foreground tab to run every minute.

This is hidden behind a feature which is disabled by default, so there
should be no changes to functionality.

Bug: 1109209
Change-Id: Icb2474b1026e2283ccc772cd3a602d46ef628dc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370655Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/master@{#803848}