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Add OPTIONAL_TRACE_EVENT{0,1,2} macros. They are equivalent to regular
TRACE_EVENTs, but they are disabled by default on the platforms where
binary size is a significant factor.

R=eseckler@chromium.org
BUG=1043616

Change-Id: Ifb4265856826423c5079c5d412fc1234db19e820
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359132
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800770}