Unset feature flag correctly in TabSwitcherMediatorUnitTest.

Change-Id: Icd86dccbea865e42373500f08fa354383150bff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079230Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745222}

chrome/android/features/tab_ui/junit/src/org/chromium/chrome/browser/tasks/tab_management/TabSwitcherMediatorUnitTest.java

@@ -195,7 +195,7 @@ public class TabSwitcherMediatorUnitTest {
     public void tearDown() {
         RecordUserAction.setDisabledForTests(false);
         RecordHistogram.setDisabledForTests(false);
-        CachedFeatureFlags.setForTesting(ChromeFeatureList.TAB_GRID_LAYOUT_ANDROID, false);
+        CachedFeatureFlags.setForTesting(ChromeFeatureList.TAB_GRID_LAYOUT_ANDROID, null);
     }
 
     @Test