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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3802 matching lines @@
     AutoLockUIResourceBitmap bitmap_lock(bitmap);
     SkBitmap source_bitmap;
     source_bitmap.setInfo(info, row_bytes);
     source_bitmap.setPixels(const_cast<uint8_t*>(bitmap_lock.GetPixels()));
 
     // This applies the scale to draw the |bitmap| into |scaled_bitmap|.
     SkBitmap scaled_bitmap;
     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);
     auto* pixels = static_cast<uint8_t*>(scaled_bitmap.getPixels());
     resource_provider_->CopyToResource(id, pixels, upload_size);
   }
 
   UIResourceData data;
   data.resource_id = id;
   data.opaque = bitmap.GetOpaque();
@@ skipping 350 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc