Worker: Refine state management of WorkerThread

This CL...

1) introduces ThreadState enum that represents a state of the worker thread,
2) stops accessing |m_globalScope| from the main thread for detecting worker
   thread initialization and instead checks ThreadState,
3) renames |m_started| and |m_terminated| with |m_requestedToStart| and
   |m_requestedToTerminate| in order to emphasize that these fields represent
   not a state of the worker thread but facts that the worker thread has been
   requested to start/terminate from the main thread, and
4) removes mention of |m_microtaskRunner| from a comment about
   |m_threadStateMutex| because the member is accessed only from the worker
   thread.

BUG=638877, 640843

Review-Url: https://codereview.chromium.org/2268183005
Cr-Commit-Position: refs/heads/master@{#414336}

third_party/WebKit/Source/core/workers/WorkerThread.h

@@ -205,13 +205,22 @@ private:
     ExitCode getExitCodeForTesting();
 
     // Accessed only on the main thread.
-    bool m_started = false;
+    bool m_requestedToStart = false;
 
     // Set on the main thread and checked on both the main and worker threads.
-    bool m_terminated = false;
-
-    // Set on the worker thread and checked on both the main and worker threads.
-    bool m_readyToShutdown = false;
+    bool m_requestedToTerminate = false;
+
+    // Represents the state of this worker thread. A caller may need to acquire
+    // a lock |m_threadStateMutex| before accessing this:
+    //   - Only the worker thread can set this with the lock.
+    //   - The worker thread can read this without the lock.
+    //   - The main thread can read this with the lock.
+    enum class ThreadState {
+        NotStarted,
+        Running,
+        ReadyToShutdown,
+    };
+    ThreadState m_threadState = ThreadState::NotStarted;
 
     // Accessed only on the worker thread.
     bool m_pausedInDebugger = false;
@@ -229,8 +238,8 @@ private:
     RefPtr<WorkerLoaderProxy> m_workerLoaderProxy;
     WorkerReportingProxy& m_workerReportingProxy;
 
-    // This lock protects |m_globalScope|, |m_terminated|, |m_readyToShutdown|,
-    // |m_runningDebuggerTask|, |m_exitCode| and |m_microtaskRunner|.
+    // This lock protects |m_globalScope|, |m_requestedToTerminate|,
+    // |m_threadState|, |m_runningDebuggerTask| and |m_exitCode|.
     Mutex m_threadStateMutex;
 
     Persistent<ConsoleMessageStorage> m_consoleMessageStorage;