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:
/chrome/browser/data_reduction_proxy/data_reduction_proxy_settings_unittest_android.cc

This CL was uploaded by git cl split.

R=mthiesse@chromium.org

Bug: 1018667
Change-Id: Ie46aeaf56588980d38c7ae4ea2fd6fc57ad0110e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916868
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715297}

chrome/browser/data_reduction_proxy/data_reduction_proxy_settings_unittest_android.cc

@@ -333,7 +333,6 @@ TEST_F(DataReductionProxySettingsAndroidTest,
 
 TEST_F(DataReductionProxySettingsAndroidTest,
        MaybeRewriteWebliteUrlWithHoldbackEnabled) {
-  base::FieldTrialList field_trial_list(nullptr);
   ASSERT_TRUE(base::FieldTrialList::CreateFieldTrial(
       "DataCompressionProxyHoldback", "Enabled"));