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Changes WebContentsImpl Loading trace event to be a nestable trace event
which means that it will show up in perfetto which doesn't support
non-nested async events.

Bug: 1142477
Change-Id: Ic8a279b5b0c99a02d0cde0a05ffcd45bfef2c827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494863
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820826}