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Add ProfileProvider to manage multiple metric collectors and register with the
various Chromium triggers.

Both the implementation and tests are mostly new.

Added rules to compile the class and the associated unit tests, but the
code will not be used in prod until a later CL that adds a perf events
collector to replace PerfProvider.

BUG=b:116527691
TEST=Unit tests pass

Change-Id: Iac745beb5f80ccfb675de581e8ccb5b7304f6114
Reviewed-on: https://chromium-review.googlesource.com/c/1357639
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615632}