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Add a new DISABLED_BY_DEFAULT trace category "xr.debug", and use
that for some timing traces that are useful when debugging scheduling
issues but a bit verbose otherwise.

Change-Id: Id72c0b6c8f7a7fab2e7428adcffa095915a61f54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468277
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816740}