Add assertHeld() to SkMutex.

src/ports/SkMutex_win.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkMutex_win_DEFINED
 #define SkMutex_win_DEFINED
 
@@ skipping 18 matching lines @@
 #  undef NOMINMAX_WAS_LOCALLY_DEFINED
 #  undef NOMINMAX
 #endif
 
 // On Windows, SkBaseMutex and SkMutex are the same thing,
 // we can't easily get rid of static initializers.
 class SkMutex {
 public:
     SkMutex() {
         InitializeCriticalSection(&fStorage);
+        SkDEBUGCODE(fOwner = 0;)
     }
 
     ~SkMutex() {
+        SkASSERT(0 == fOwner);
         DeleteCriticalSection(&fStorage);
     }
 
     void acquire() {
         EnterCriticalSection(&fStorage);
+        SkDEBUGCODE(fOwner = GetCurrentThreadId();)
     }
 
     void release() {
+        this->assertHeld();
+        SkDEBUGCODE(fOwner = 0;)
         LeaveCriticalSection(&fStorage);
     }
 
+    void assertHeld() {
+        SkASSERT(GetCurrentThreadId() == fOwner);
+    }
+
 private:
     SkMutex(const SkMutex&);
     SkMutex& operator=(const SkMutex&);
 
     CRITICAL_SECTION fStorage;
+    SkDEBUGCODE(DWORD fOwner;)
 };
 
 typedef SkMutex SkBaseMutex;
 
 // Windows currently provides no documented means of POD initializing a CRITICAL_SECTION.
 #define SK_DECLARE_STATIC_MUTEX(name) static SkBaseMutex name
 #define SK_DECLARE_GLOBAL_MUTEX(name) SkBaseMutex name
 
 #endif