Restrict SW Renderer clear to scissor

cc/output/software_renderer.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/software_renderer.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/base/math_util.h"
 #include "cc/base/render_surface_filters.h"
@@ skipping 134 matching lines @@
             current_canvas_->imageInfo().width());
   DCHECK_EQ(current_canvas_->getDeviceClipBounds().height(),
             current_canvas_->imageInfo().height());
   current_canvas_->clipRect(gfx::RectToSkRect(rect));
   current_canvas_->setMatrix(current_matrix);
 }
 
 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() {
   if (current_frame()->current_render_pass->has_transparent_background) {
     ClearCanvas(SkColorSetARGB(0, 0, 0, 0));
   } else {
 #ifndef NDEBUG
     // On DEBUG builds, opaque render passes are cleared to blue
     // to easily see regions that were not drawn on the screen.
     ClearCanvas(SkColorSetARGB(255, 0, 0, 255));
@@ skipping 555 matching lines @@
       ApplyImageFilter(filter.get(), quad, backdrop_bitmap, nullptr);
 
   if (!filter_backdrop_image)
     return nullptr;
 
   return filter_backdrop_image->makeShader(content_tile_mode, content_tile_mode,
                                            &filter_backdrop_transform);
 }
 
 }  // namespace cc