Implement DrawQuad StructTraits

cc/quads/render_pass_draw_quad.h

Index: cc/quads/render_pass_draw_quad.h
diff --git a/cc/quads/render_pass_draw_quad.h b/cc/quads/render_pass_draw_quad.h
index 94461a1a328e3ec4cea8d6857aa16446eec65854..97dd4f24db48e168e7f9706fec1e4b237ee22f13 100644
--- a/cc/quads/render_pass_draw_quad.h
+++ b/cc/quads/render_pass_draw_quad.h
@@ -19,6 +19,8 @@ namespace cc {
 
 class CC_EXPORT RenderPassDrawQuad : public DrawQuad {
  public:
+  static const size_t kMaskResourceIdIndex = 0;
+
   RenderPassDrawQuad();
   RenderPassDrawQuad(const RenderPassDrawQuad& other);
   ~RenderPassDrawQuad() override;
@@ -74,8 +76,6 @@ class CC_EXPORT RenderPassDrawQuad : public DrawQuad {
   static const RenderPassDrawQuad* MaterialCast(const DrawQuad*);
 
  private:
-  static const size_t kMaskResourceIdIndex = 0;
-
   void ExtendValue(base::trace_event::TracedValue* value) const override;
 };