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This flag specifies whether to target the platform's copy of the HWASan
runtime, rather than one bundled with the application.

I decided to make this a build flag rather than change the default because
there are use cases for both settings.

- Builds with hwasan_platform=true may only be used on HWASANified builds of
  the platform.
- Builds with hwasan_platform=false are only expected to work on
  non-HWSANified builds of the platform. They may work in HWASANified builds,
  but this is not guaranteed because the compiler is allowed to target the
  bundled runtime library which may have a different ABI to the platform's
  copy.

Moreover, hwasan_platform=false requires the compiler to emit additional
code into each function which can hurt performance and binary size.

Bug: 916828
Change-Id: I45c9ccdc0a5cf1b3c6a729263204a023b40ba15f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642787
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666092}