Refactor TimeBase to post tasks. Workers to use real Idle tasks.

Source/core/animation/CompositorAnimationsTestHelper.h

Index: Source/core/animation/CompositorAnimationsTestHelper.h
diff --git a/Source/core/animation/CompositorAnimationsTestHelper.h b/Source/core/animation/CompositorAnimationsTestHelper.h
index 31c0f09ab9561789275e1dfd63b48e9da0ed98b2..3c933b8fd07ba8475ac13fda2b76013213c204cd 100644
--- a/Source/core/animation/CompositorAnimationsTestHelper.h
+++ b/Source/core/animation/CompositorAnimationsTestHelper.h
@@ -142,7 +142,7 @@ public:
 private:
     class PlatformProxy : public Platform {
     public:
-        PlatformProxy(WebCompositorSupportMock** compositor) : m_compositor(compositor) { }
+        PlatformProxy(WebCompositorSupportMock** compositor) : m_oldPlatform(blink::Platform::current()), m_compositor(compositor) { }
 
         virtual void cryptographicallyRandomValues(unsigned char* buffer, size_t length) { ASSERT_NOT_REACHED(); }
         const unsigned char* getTraceCategoryEnabledFlag(const char* categoryName) override
@@ -151,7 +151,13 @@ private:
             return &tracingIsDisabled;
         }
 
+        WebThread* currentThread() override
+        {
+            return m_oldPlatform->currentThread();
+        }
+
     private:
+        blink::Platform* m_oldPlatform; // NOT OWNED
         WebCompositorSupportMock** m_compositor;
         virtual WebCompositorSupport* compositorSupport() override { return *m_compositor; }
     };