Add assertHeld() to SkMutex.

src/ports/SkMutex_win.h

Index: src/ports/SkMutex_win.h
diff --git a/src/ports/SkMutex_win.h b/src/ports/SkMutex_win.h
index f4b8234e85d70305ee28deb634158ae703a0734d..d12fd033f3d6d6a457d1b9022987572f8be9690f 100644
--- a/src/ports/SkMutex_win.h
+++ b/src/ports/SkMutex_win.h
@@ -36,25 +36,35 @@ 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;