Purge Skia objects from GL caches as needed.

Change-Id: I754c671cf790ad5ae8bf047ad328034217da4ecc

include/utils/Singleton.h

@@ -37,6 +37,11 @@ public:
         }
         return *instance;
     }
+
+    static bool hasInstance() {
+        Mutex::Autolock _l(sLock);
+        return sInstance != 0;
+    }
     
 protected:
     ~Singleton() { };