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This uses the Clang profiling config during the instrumentation step of
a PGO build. This allows getting the profiling data out of the sandboxed
processes. This is not an ideal end state, ideally the coverage
configuration should be refactored and things common to the two
configurations should be moved to a new base config.

This requires to use the new version of goma, see go/ma-rbe-dogfood
(see crbug.com/990981#c26 for more details).

To build a PGInstrumented build (first step to do a PGO build) compile
with the following GN args:
  is_component_build = false
  is_debug = false
  chrome_pgo_phase = 1
  use_goma = true

Bug: 1056189
Change-Id: I717efdcbb6993ad574acce4c0f91eb5fb5bf3972
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076424
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750258}