Add assertHeld() to SkMutex.

include/core/SkThread.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkThread_DEFINED
 #define SkThread_DEFINED
 
@@ skipping 65 matching lines @@
  *  Only needs to be implemented for T which can be atomically written.
  */
 template <typename T> void sk_release_store(T*, T);
 
 #include SK_BARRIERS_PLATFORM_H
 
 /** SK_MUTEX_PLATFORM_H must provide the following (or equivalent) declarations.
 
 class SkBaseMutex {
 public:
-    void acquire();
-    void release();
+    void acquire();     // Block until this thread owns the mutex.
+    void release();     // Assuming this thread owns the mutex, release it.
+    void assertHeld();  // If SK_DEBUG, assert this thread owns the mutex.
 };
 
 class SkMutex : SkBaseMutex {
 public:
     SkMutex();
     ~SkMutex();
 };
 
 #define SK_DECLARE_STATIC_MUTEX(name) static SkBaseMutex name = ...
 #define SK_DECLARE_GLOBAL_MUTEX(name) SkBaseMutex name = ...
@@ skipping 23 matching lines @@
     }
 
     /** If the mutex has not been released, release it now. */
     void release() {
         if (fMutex) {
             fMutex->release();
             fMutex = NULL;
         }
     }
 
+    /** Assert that we're holding the mutex. */
+    void assertHeld() {
+        SkASSERT(fMutex);
+        fMutex->assertHeld();
+    }
+
 private:
     SkBaseMutex* fMutex;
 };
 #define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
 
 #endif