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Previously GWP-ASan only exposed separate flags for whether it was
enabled for a given allocator. Create a convenience buildflag for places
when we need to check whether GWP-ASan is enabled for any allocator and
use it where appropriate.

Bug: 973167
Change-Id: I76f8ee2c258279ebd07e3b6f72651498358a95da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671866
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672200}