Create debug only SkSingleOwner

include/private/SkMutex.h

Index: include/private/SkMutex.h
diff --git a/include/private/SkMutex.h b/include/private/SkMutex.h
index 8c78e1205c8796261be4b19468338a00af8afdc5..db8aefe9f580fba3df0ace5c7396159cc92394aa 100644
--- a/include/private/SkMutex.h
+++ b/include/private/SkMutex.h
@@ -54,6 +54,46 @@ struct SkBaseMutex {
     SkDEBUGCODE(SkThreadID fOwner;)
 };
 
+#ifdef SK_DEBUG
+// This is a slow recursive mutex for debug purposes only
+struct SkRecursiveMutex {
+    SkRecursiveMutex () {
+        fSemaphore = SK_MUTEX_SEMAPHORE_INIT;
+        fOwner = kIllegalThreadID;
+        fAcquireCount = 0;
+    }
+    ~SkRecursiveMutex () { fSemaphore.deleteSemaphore(); }
+
+    void acquire() {
+        if (fOwner != SkGetThreadID()) {
+            fSemaphore.wait();
+        }
+        fAcquireCount++;
+        fOwner = SkGetThreadID();
+    }
+
+    void release() {
+        this->assertHeld();
+        if (1 == fAcquireCount--) {
+            fOwner = kIllegalThreadID;
+            fSemaphore.signal();
+        }
+    }
+
+    void assertHeld() {
+        SkASSERT(fOwner == SkGetThreadID());
+    }
+
+    void assertNotHeld() {
+        SkASSERT(kIllegalThreadID == fOwner);
+    }
+
+    SkBaseSemaphore fSemaphore;
+    SkThreadID fOwner;
+    uint32_t fAcquireCount;
+};
+#endif
+
 // This needs to use subclassing instead of encapsulation to make SkAutoMutexAcquire to work.
 class SkMutex : public SkBaseMutex {
 public: