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With the addition of site isolation, we now can have render
processes that are not assigned to a top level page, hence
they do not get a descriptive title in tracing.

This CL adds information about all the hosted subframes to
the process label in tracing.

Bug: chromium:828072
Change-Id: Ia6f617dc936f0c92183684faee39c65447bfd37c
Reviewed-on: https://chromium-review.googlesource.com/990553Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Fadi Meawad <fmeawad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547700}