Add assertHeld() to SkMutex.

include/core/SkThread.h

Index: include/core/SkThread.h
diff --git a/include/core/SkThread.h b/include/core/SkThread.h
index 3038e2d9542325503295f7603fd60b45b4fca80f..4f7f326097297d3e54af718073ccce9378f543d6 100644
--- a/include/core/SkThread.h
+++ b/include/core/SkThread.h
@@ -83,8 +83,9 @@ template <typename T> void sk_release_store(T*, T);
 
 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 {
@@ -128,6 +129,12 @@ public:
         }
     }
 
+    /** Assert that we're holding the mutex. */
+    void assertHeld() {
+        SkASSERT(fMutex);
+        fMutex->assertHeld();
+    }
+
 private:
     SkBaseMutex* fMutex;
 };