Do not instantiate base::FieldTrialList explicitly in tests

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:
/components/flags_ui

This CL was uploaded by git cl split.

[email protected]

Bug: 1018667
Change-Id: I3b1e2d336fedff036b4e9e104164e723ae24f57e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917526
Auto-Submit: Mikel Astiz <[email protected]>
Reviewed-by: Alexei Svitkine <[email protected]>
Commit-Queue: Alexei Svitkine <[email protected]>
Cr-Commit-Position: refs/heads/master@{#716212}
1 file changed