Refactor OverlayStrategySandwich and OverlayStrategyCommon

cc/output/overlay_candidate.cc

Index: cc/output/overlay_candidate.cc
diff --git a/cc/output/overlay_candidate.cc b/cc/output/overlay_candidate.cc
index 5ecdc32ed3602e8095e7876e57778ea59db0d154..4e6a36b0239a15229e99b61e03b117d903535eca 100644
--- a/cc/output/overlay_candidate.cc
+++ b/cc/output/overlay_candidate.cc
@@ -211,6 +211,18 @@ bool OverlayCandidate::FromDrawQuad(const DrawQuad* quad,
 }
 
 // static
+bool OverlayCandidate::IsInvisibleQuad(const DrawQuad* quad) {
+  if (quad->material == DrawQuad::SOLID_COLOR) {
+    SkColor color = SolidColorDrawQuad::MaterialCast(quad)->color;
+    float opacity = quad->shared_quad_state->opacity;
+    float alpha = (SkColorGetA(color) * (1.0f / 255.0f)) * opacity;
+    return quad->ShouldDrawWithBlending() &&
+           alpha < std::numeric_limits<float>::epsilon();
+  }
+  return false;
+}
+
+// static
 bool OverlayCandidate::FromTextureQuad(const TextureDrawQuad* quad,
                                        OverlayCandidate* candidate) {
   if (!quad->allow_overlay())