cc: Don't perform image analysis if the DisplayItemList has no images.

cc/paint/transform_display_item.cc

 // Copyright 2014 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/paint/transform_display_item.h"
 
 namespace cc {
 
 TransformDisplayItem::TransformDisplayItem(const gfx::Transform& transform)
 
-    : DisplayItem(TRANSFORM), transform(transform) {}
+    : DisplayItem(TRANSFORM), transform(transform) {
+  // The underlying SkMatrix in gfx::Transform is not thread-safe, unless
+  // getType() has been called.
+  this->transform.matrix().getType();

[Khushal, 2017/05/04 20:58:17]

So looks like the IsIdentity check when rastering the transform display item is
inadvertently causing a race when coupled with this change. Yay mutable!
Does this look like a reasonable way to avoid that?

[enne (OOO), 2017/05/04 21:02:27]

Yeah, that's what I had to do in PaintOpBuffer as well.  Good thing we've got
oodles of layers of display lists where we have to repeat the same patterns at
every level.

+}
 
 TransformDisplayItem::~TransformDisplayItem() = default;
 
 EndTransformDisplayItem::EndTransformDisplayItem()
     : DisplayItem(END_TRANSFORM) {}
 
 EndTransformDisplayItem::~EndTransformDisplayItem() = default;
 
 }  // namespace cc