cc: Partial tile update for one-copy raster.

cc/resources/raster_source_helper.cc

Index: cc/resources/raster_source_helper.cc
diff --git a/cc/resources/raster_source_helper.cc b/cc/resources/raster_source_helper.cc
index 34436e4a3f04c4d33cdc0afc28885581c4472a3b..af8402fa2bbbb7bd313e49d7a4c122fdb07a6262 100644
--- a/cc/resources/raster_source_helper.cc
+++ b/cc/resources/raster_source_helper.cc
@@ -14,16 +14,29 @@ namespace cc {
 
 void RasterSourceHelper::PrepareForPlaybackToCanvas(
     SkCanvas* canvas,
-    const gfx::Rect& canvas_rect,
+    const gfx::Rect& canvas_bitmap_rect,
+    const gfx::Rect& canvas_playback_rect,
     const gfx::Rect& source_rect,
     float contents_scale,
     SkColor background_color,
     bool clear_canvas_with_debug_color,
     bool requires_clear) {
-  canvas->discard();
+  bool partial_update = canvas_bitmap_rect != canvas_playback_rect;
+  gfx::Rect canvas_rect =
+      canvas_playback_rect - canvas_bitmap_rect.OffsetFromOrigin();
+
+  if (!partial_update)
+    canvas->discard();
   if (clear_canvas_with_debug_color) {
     // Any non-painted areas in the content bounds will be left in this color.
-    canvas->clear(DebugColors::NonPaintedFillColor());
+    if (!partial_update) {
+      canvas->clear(DebugColors::NonPaintedFillColor());
+    } else {
+      canvas->save();
+      canvas->clipRect(gfx::RectToSkRect(canvas_rect));
+      canvas->drawColor(DebugColors::NonPaintedFillColor());
+      canvas->restore();
+    }
   }
 
   // If this raster source has opaque contents, it is guaranteeing that it will
@@ -33,7 +46,14 @@ void RasterSourceHelper::PrepareForPlaybackToCanvas(
     TRACE_EVENT_INSTANT0("cc", "SkCanvas::clear", TRACE_EVENT_SCOPE_THREAD);
     // Clearing is about ~4x faster than drawing a rect even if the content
     // isn't covering a majority of the canvas.
-    canvas->clear(SK_ColorTRANSPARENT);
+    if (!partial_update) {
+      canvas->clear(SK_ColorTRANSPARENT);
+    } else {
+      canvas->save();
+      canvas->clipRect(gfx::RectToSkRect(canvas_rect));
+      canvas->drawColor(SK_ColorTRANSPARENT);
+      canvas->restore();
+    }
   } else {
     // Even if completely covered, for rasterizations that touch the edge of the
     // layer, we also need to raster the background color underneath the last