OnByDefault fix for content_browsertests.

This CL prepares content_browsertests for when the feature is turned
on by default. The fix here is make DoesScrollbarScrollOnMainThread
rely on the base::Feature rather than just returning a hardcoded
value.

Bug: 1049814
Change-Id: I0055df6d1abcb31b928a7985dad7eecf7882ad7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042762Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#740749}

content/browser/renderer_host/input/scroll_latency_browsertest.cc

@@ -272,9 +272,17 @@ class ScrollLatencyScrollbarBrowserTest : public ScrollLatencyBrowserTest {
   void SetUpCommandLine(base::CommandLine* command_line) override {
     ScrollLatencyBrowserTest::SetUpCommandLine(command_line);
     command_line->AppendSwitch(::switches::kDisableSmoothScrolling);
-    // Disable kOverlayScrollbar since overlay scrollbars are not
-    // hit-testable (thus input is not routed to scrollbars).
-    scoped_feature_list_.InitAndDisableFeature({features::kOverlayScrollbar});
+
+    // The following features need to be disabled:
+    // - kOverlayScrollbar since overlay scrollbars are not hit-testable (thus
+    // input is not routed to scrollbars).
+    // - kCompositorThreadedScrollbarScrolling since this feature is already
+    // tested by ScrollLatencyCompositedScrollbarBrowserTest. Hence, this
+    // current test can be used exclusively to test the main thread scrollbar
+    // path.
+    scoped_feature_list_.InitWithFeaturesAndParameters(
+        {}, {features::kOverlayScrollbar,
+             features::kCompositorThreadedScrollbarScrolling});
   }
 
   ~ScrollLatencyScrollbarBrowserTest() override {}