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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 87 matching lines @@
   int bit_fields;
   void* pointers[11];
   LayoutUnit layout_units[4];
   IntSize size;
   Persistent<PaintLayerScrollableArea> scrollable_area;
   struct {
     IntSize size;
     void* pointer;
     LayoutRect rect;
   } previous_paint_status;
+  uint64_t unique_id;
 };
 
 static_assert(sizeof(PaintLayer) == sizeof(SameSizeAsPaintLayer),
               "PaintLayer should stay small");
 
 bool IsReferenceClipPath(const ClipPathOperation* clip_operation) {
   return clip_operation &&
          clip_operation->GetType() == ClipPathOperation::REFERENCE;
 }
 
@@ skipping 42 matching lines @@
       self_painting_status_changed_(false),
       layout_object_(layout_object),
       parent_(0),
       previous_(0),
       next_(0),
       first_(0),
       last_(0),
       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();
 
   UpdateScrollableArea();
 }
 
 PaintLayer::~PaintLayer() {
   if (rare_data_ && rare_data_->resource_info) {
     const ComputedStyle& style = GetLayoutObject().StyleRef();
@@ skipping 3161 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->EnclosingLayer());
 }
 #endif