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This change kicks off native library loading as early as possible in a
separate thread, to reduce startup latency. There may be a few
activities that don't actually need native libs, but the vast majority
of the time we end up loading the native lib, and this seems like a good
trade off for the perf improvements.

Pinpoint results for messageloop_start_time in mobile.startup:
Android Go:
Cold start: -11.3% https://pinpoint-dot-chromeperf.appspot.com/job/10e6fbf3520000
Warm start: -23%: https://pinpoint-dot-chromeperf.appspot.com/job/1227a4cd520000

Pixel 2:
Cold start: -0.3% https://pinpoint-dot-chromeperf.appspot.com/job/11822b23520000
Warm start: -1.7% https://pinpoint-dot-chromeperf.appspot.com/job/154a85cb520000

Bug: 1148803
Change-Id: I70256509cad47c20ec047222ba7bd0f576abe548
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535732
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827862}