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Per the results of the beta experiment, it reduces memory footprint on desktop
and Android, doesn't regress responsiveness, and doesn't cause crashes.

From the current dev/beta Finch experiment (full results here, google internal:
https://docs.google.com/document/d/13mLZCzn8tcu3HzLClHT5PZ13GZ3R1a1qyo5fTHMPZmo/edit)
- Windows: 1.3MB median PrivateMemoryFootprint savings, 8.8MB @95th percentile
- Android: 0.7MB @median, 4.4MB @95th percentile

In both cases, foreground CPU cost is <0.1% @95th percentile. Savings above are
per renderer, for all renderers.
On popular websites, savings are higher, e.g. 15MB median PMF savings on
facebook.com, desktop.

As a consequence, enabling it by default.

Bug: 924164, 951773
Change-Id: I4742cbf053b08a259774c453b555e22ebc44de1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556852
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650432}