Grant independence to SkBaseMutex on Windows.

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 d12fd033f3d6d6a457d1b9022987572f8be9690f..d84b0e42fd0da5fe0607544c6453001283d62a03 100644
--- a/src/ports/SkMutex_win.h
+++ b/src/ports/SkMutex_win.h
@@ -31,15 +31,17 @@
 #endif
 
 // On Windows, SkBaseMutex and SkMutex are the same thing,
-// we can't easily get rid of static initializers.
-class SkMutex {
+// we can't easily get rid of static initializers. However,
+// we preserve the same inheritance pattern as other platforms
+// so that we can forward-declare cleanly.
+struct SkBaseMutex {
 public:
-    SkMutex() {
+    SkBaseMutex() {
         InitializeCriticalSection(&fStorage);
         SkDEBUGCODE(fOwner = 0;)
     }
 
-    ~SkMutex() {
+    ~SkBaseMutex() {
         SkASSERT(0 == fOwner);
         DeleteCriticalSection(&fStorage);
     }
@@ -59,15 +61,16 @@ public:
         SkASSERT(GetCurrentThreadId() == fOwner);
     }
 
-private:
-    SkMutex(const SkMutex&);
-    SkMutex& operator=(const SkMutex&);
-
+protected:
     CRITICAL_SECTION fStorage;
     SkDEBUGCODE(DWORD fOwner;)
+
+private:
+    SkBaseMutex(const SkBaseMutex&);
+    SkBaseMutex& operator=(const SkBaseMutex&);
 };
 
-typedef SkMutex SkBaseMutex;
+class SkMutex : public SkBaseMutex { };
 
 // Windows currently provides no documented means of POD initializing a CRITICAL_SECTION.
 #define SK_DECLARE_STATIC_MUTEX(name) static SkBaseMutex name