compositor-worker: Get the thread to run compositor-workers from the Platform.

Source/core/workers/WorkerThreadTest.cpp

Index: Source/core/workers/WorkerThreadTest.cpp
diff --git a/Source/core/workers/WorkerThreadTest.cpp b/Source/core/workers/WorkerThreadTest.cpp
index 14549166e531cd3b6f08bab8c897ef3212979b5f..6f686fbe2af4c44feb077ddf46c4f86b993cef01 100644
--- a/Source/core/workers/WorkerThreadTest.cpp
+++ b/Source/core/workers/WorkerThreadTest.cpp
@@ -105,9 +105,9 @@ public:
     ~WorkerThreadForTest() override { }
 
     // WorkerThread implementation:
-    WebThreadSupportingGC& backingThread() override
+    WebThread& backingThread() override
     {
-        return *m_thread;
+        return m_thread->platformThread();
     }
 
     MOCK_METHOD1(doIdleGc, bool(double deadlineSeconds));
@@ -324,7 +324,7 @@ TEST_F(WorkerThreadTest, GcDoesNotOccurWhenNotIdle)
     start();
     waitForInit();
 
-    WebScheduler* scheduler = m_workerThread->backingThread().platformThread().scheduler();
+    WebScheduler* scheduler = m_workerThread->backingThread().scheduler();
 
     // Post a repeating task that should prevent any GC from happening.
     scheduler->postLoadingTask(FROM_HERE, new RepeatingTask(scheduler, completion.get()));