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

cc/paint/transform_display_item.cc

Index: cc/paint/transform_display_item.cc
diff --git a/cc/paint/transform_display_item.cc b/cc/paint/transform_display_item.cc
index 6cc3e7f2d1edc2e56d0da467eb877d152c855b30..54c034e8502dbd4be386410dd1075ed9811fbc26 100644
--- a/cc/paint/transform_display_item.cc
+++ b/cc/paint/transform_display_item.cc
@@ -8,7 +8,11 @@ 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;