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The goal is to support having a BEST_EFFORT execution fence prior to
main message loop start in BrowserMainLoop, and a fence when visible
tabs are loading in //chrome/browser/resource_coordinator. The lifetime
of both fences would overlap. The reason for not having a single fence
is that the first fence is controlled by //content whereas the second
fence is controlled by //chrome (precise tracking of whether a tab is
loading is a //chrome feature).

Bug: 839110, 1016825
Change-Id: Icaceee4cf9e5a1d4c52f9bb91e69e9f8125cee2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872807
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711688}