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This CL adds a new UKM Document.OutliveTimeAfterShutdown, that is
recorded when a Document object survives 5, 10, 20 or 50 garbage
collections after detached. If a document outlives such long time, the
document might be leaked. The UKM would be very useful to know where such
leaky documents exist and to fix them.

Design doc: https://docs.google.com/document/d/1fbs5smdd-pBLLMpq7u8EkyddZILtI7CZPJlo_AA1kak/edit?usp=sharing

Bug: 757374
Change-Id: Idc05b03f625db11ab54c5203d18344b1ca72b4eb
Reviewed-on: https://chromium-review.googlesource.com/647050
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505275}