The flag is enabled by default and provides a quick way to switch
between the old RetainerInfo and the new EmbedderGraph snapshoting
in local testing.

Bug: chromium:749490
Change-Id: I36406597a289090879cfa5051037c8cf35988e59
Reviewed-on: https://chromium-review.googlesource.com/897532Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51045}