Add ASSERT(isMainThread()) to WorkerThread::start() and ::terminate()

Source/core/workers/WorkerThread.cpp

Index: Source/core/workers/WorkerThread.cpp
diff --git a/Source/core/workers/WorkerThread.cpp b/Source/core/workers/WorkerThread.cpp
index ee06a089949d8c1f193bb1558b17f2e439fa86d6..99bb64e04f4d6bb50823d8e15187a3ee5c7f20b3 100644
--- a/Source/core/workers/WorkerThread.cpp
+++ b/Source/core/workers/WorkerThread.cpp
@@ -177,6 +177,8 @@ WorkerThread::~WorkerThread()
 
 void WorkerThread::start(PassOwnPtr<WorkerThreadStartupData> startupData)
 {
+    ASSERT(isMainThread());
+
     if (m_started)
         return;
 
@@ -307,6 +309,8 @@ bool WorkerThread::terminated()
 
 void WorkerThread::terminateInternal()
 {
+    ASSERT(isMainThread());
+
     // Protect against this method, initialize() or termination via the global scope racing each other.
     MutexLocker lock(m_threadStateMutex);