SkiaRenderer: Move RPDQ effect preparation into reusable function

This is a preparatory step to make it easy to apply RPDQ effects
from any of the DrawXQuad() functions in SkiaRenderer, so that
bypassing a RenderPass doesn't depend on the underlying quad's material.

Bug: 1013735
Change-Id: I81e55c7f469673ba618abc54ebf72feff88e0e22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860294
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707095}

components/viz/service/display/skia_renderer.h

@@ -104,6 +104,11 @@ class VIZ_SERVICE_EXPORT SkiaRenderer : public DirectRenderer {
   void PrepareCanvas(const base::Optional<gfx::Rect>& scissor_rect,
                      const base::Optional<gfx::RRectF>& rounded_corner_bounds,
                      const gfx::Transform* cdt);
+  // Further modify the canvas or draw parameters as needed to apply the effects
+  // represented by |rpdq_params|.  |content_paint| must be a non-null pointer.
+  void PrepareCanvasForRPDQ(const DrawRPDQParams& rpdq_params,
+                            DrawQuadParams* params,
+                            SkPaint* content_paint);
 
   // The returned DrawQuadParams can be modified by the DrawX calls that accept
   // params so that they can apply explicit data transforms before sending to