Restrict SW Renderer clear to scissor

cc/output/software_renderer.cc

Index: cc/output/software_renderer.cc
diff --git a/cc/output/software_renderer.cc b/cc/output/software_renderer.cc
index 34eda97f5d477b5b878000184fd1a0346eaa9fb2..6fc30a038f0da1314d53fbaf443ae6158a24dd57 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -152,7 +152,16 @@ void SoftwareRenderer::SetClipRect(const gfx::Rect& rect) {
 void SoftwareRenderer::ClearCanvas(SkColor color) {
   if (!current_canvas_)
     return;
-  current_canvas_->clear(color);
+
+  if (is_scissor_enabled_) {
+    // The same paint used by SkCanvas::clear, but applied to the scissor rect.
+    SkPaint clear_paint;
+    clear_paint.setColor(color);
+    clear_paint.setBlendMode(SkBlendMode::kSrc);
+    current_canvas_->drawRect(gfx::RectToSkRect(scissor_rect_), clear_paint);
+  } else {
+    current_canvas_->clear(color);
+  }
 }
 
 void SoftwareRenderer::ClearFramebuffer() {