scroll: Replace the default timer task runner with one for testing

This is part of efforts to replace non-per-frame task runners with other
appropriate task runners in the renderer.

Bug: 804694
Change-Id: Ibcb96b6461fda0d777edcb961bc21294fcc5cf23
Reviewed-on: https://chromium-review.googlesource.com/897308Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534026}

third_party/WebKit/Source/platform/scroll/ScrollableAreaTest.cpp

@@ -34,7 +34,10 @@ class ScrollbarThemeWithMockInvalidation : public ScrollbarThemeMock {
 
 }  // namespace
 
-using ScrollableAreaTest = ::testing::Test;
+class ScrollableAreaTest : public ::testing::Test {
+ private:
+  base::MessageLoop message_loop_;
+};
 
 TEST_F(ScrollableAreaTest, ScrollAnimatorCurrentPositionShouldBeSync) {
   ScopedTestingPlatformSupport<TestingPlatformSupportWithMockScheduler>

third_party/WebKit/Source/platform/scroll/ScrollbarTestSuite.h

@@ -15,6 +15,7 @@
 #include "platform/wtf/PtrUtil.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebThread.h"
+#include "public/platform/scheduler/test/renderer_scheduler_test_support.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace blink {
@@ -96,7 +97,7 @@ class MockScrollableArea : public GarbageCollectedFinalized<MockScrollableArea>,
   }
 
   scoped_refptr<base::SingleThreadTaskRunner> GetTimerTaskRunner() const final {
-    return Platform::Current()->CurrentThread()->Scheduler()->TimerTaskRunner();
+    return blink::scheduler::GetSingleThreadTaskRunnerForTesting();
   }
 
   PlatformChromeClient* GetChromeClient() const override {

third_party/WebKit/Source/platform/scroll/ScrollbarThemeAuraTest.cpp

@@ -29,7 +29,10 @@ class ScrollbarThemeAuraButtonOverride final : public ScrollbarThemeAura {
 
 }  // namespace
 
-using ScrollbarThemeAuraTest = ::testing::Test;
+class ScrollbarThemeAuraTest : public ::testing::Test {
+ private:
+  base::MessageLoop message_loop_;
+};
 
 TEST_F(ScrollbarThemeAuraTest, ButtonSizeHorizontal) {
   ScopedTestingPlatformSupport<TestingPlatformSupportWithMockScheduler>

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlayTest.cpp

@@ -12,7 +12,10 @@ namespace blink {
 using ::testing::NiceMock;
 using ::testing::Return;
 
-using ScrollbarThemeOverlayTest = ::testing::Test;
+class ScrollbarThemeOverlayTest : public ::testing::Test {
+ private:
+  base::MessageLoop message_loop_;
+};
 
 TEST_F(ScrollbarThemeOverlayTest, PaintInvalidation) {
   ScopedTestingPlatformSupport<TestingPlatformSupportWithMockScheduler>