Clear the scaled bitmap before drawing into it for huge UIResources.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index f46d3cddfba829d2c71428b1f699eddaea703af4..2ec8f709dc2add212eb5f63ffc57fd2fa8bf4ee9 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3820,6 +3820,11 @@ void LayerTreeHostImpl::CreateUIResource(UIResourceId uid,
     scaled_bitmap.allocN32Pixels(upload_size.width(), upload_size.height());
     SkCanvas scaled_canvas(scaled_bitmap);
     scaled_canvas.scale(canvas_scale_x, canvas_scale_y);
+    // The |canvas_scale_x| and |canvas_scale_y| may have some floating point
+    // error for large enough values, causing pixels on the edge to be not
+    // fully filled by drawBitmap(), so we ensure they start empty. (See
+    // crbug.com/642011 for an example.)
+    scaled_canvas.clear(SK_ColorTRANSPARENT);
     scaled_canvas.drawBitmap(source_bitmap, 0, 0);
 
     SkAutoLockPixels scaled_bitmap_lock(scaled_bitmap);