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The test suite already instantiates a FieldTrialList as per
https://chromium-review.googlesource.com/c/chromium/src/+/1883567 so
it's no longer necessary to do so explicitly in tests.

This patch addresses unit tests under:
/chrome/browser/metrics/perf

This CL was uploaded by git cl split.

R=gmx@chromium.org

Bug: 1018667
Change-Id: I8ad604b8c375a8fbe4f3ba49c1f51940fe2adbb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917504
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715760}