Bug 1117772 - Return false from PluginInstanceParent::CreateBackground; r=baku

clang emits the following warning on this code, which is treated as an
error:
error: implicit conversion of nullptr constant to 'bool' [-Werror,-Wnull-conversion]

dom/plugins/ipc/PluginInstanceParent.cpp

@@ -813,7 +813,7 @@ PluginInstanceParent::CreateBackground(const nsIntSize& aSize)
             gfxImageFormat::RGB24);
     return !!mBackground;
 #else
-    return nullptr;
+    return false;
 #endif
 }