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When test tracing is enabled, by default browser tests generate a file
name based on the test name. However, when a test a parametrised, it has
slashes in its full name (e.g. All/Foo.Bar/0).

Replace slashes with underscores to ensure that we don't fail to
actually write a trace file to the file.

R=sky@chromium.org

Change-Id: I8073e0dbd925b9c35e97dd7f6726b6b5b9210d7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207376
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770777}