Fix Android compiler warnings.

Review URL: https://codereview.appspot.com/7040053

git-svn-id: http://skia.googlecode.com/svn/trunk@7014 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/gpu/GrBackendEffectFactory.h b/include/gpu/GrBackendEffectFactory.h
index 2bfefb9..437bcdd 100644
--- a/include/gpu/GrBackendEffectFactory.h
+++ b/include/gpu/GrBackendEffectFactory.h
@@ -63,6 +63,7 @@
     GrBackendEffectFactory() {
         fEffectClassID = kIllegalEffectClassID;
     }
+    virtual ~GrBackendEffectFactory() {}
 
     static EffectKey GenID() {
         GR_DEBUGCODE(static const int32_t kClassIDBits = 8 * sizeof(EffectKey) -