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This CL is adding trace events to look closer on some slow cases
happening in the browser process. Traces will be collected via
slow-reports and an investigation will be continued based on results of
these events.

R=fdoray@chromium.org, ccameron@chromium.org

Bug: 894459
Change-Id: I2e3c37e9a42ceef9ca9aea5ec94a125db247aa77
Reviewed-on: https://chromium-review.googlesource.com/c/1277825
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599205}