[SPv1] Always set a CompositorElementId on main graphics layers; use PaintLayer id.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 3c6ca36ef8bf665bf84fc2ce6d74c2fb049ff6de..e9bd7123e662fa7aa84d0b54321dc96f1b34322a 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -105,6 +105,7 @@ struct SameSizeAsPaintLayer : DisplayItemClient {
     void* pointer;
     LayoutRect rect;
   } previous_paint_status;
+  uint64_t unique_id;
 };
 
 static_assert(sizeof(PaintLayer) == sizeof(SameSizeAsPaintLayer),
@@ -167,6 +168,8 @@ PaintLayer::PaintLayer(LayoutBoxModelObject& layout_object)
       static_inline_position_(0),
       static_block_position_(0),
       ancestor_overflow_layer_(nullptr) {
+  static PaintLayerId paint_layer_id_counter = 0;
+  unique_id_ = ++paint_layer_id_counter;
   UpdateStackingNode();
 
   is_self_painting_layer_ = ShouldBeSelfPaintingLayer();