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Move additional logic to the base class MetricCollector for reuse between
different collector implementations, e.g. writing outcomes to UMA histograms,
removal of unknown fields from protos, saving profiles to the local cache.
This change enables making two additional fields private.

Moved tests for discarding unknown fields and for saving perf data and perf
stat protos, to the metric collector unit tests.

BUG=b:110205489
TEST=Unit tests pass

Change-Id: I8877e9abc6ee8d89e6be391063d3ddf7ba231bc0
Reviewed-on: https://chromium-review.googlesource.com/c/1392352Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619836}