[BlinkGenPropertyTrees] Add simple layer list logging

This patch adds layer list logging. To use, start content shell with:
  --vmodule=layer_tree_host=3

Sample output:
  layer id 11
    element_id: (0)
    bounds: 800x600
    opacity: 1
    position: 0.000000,0.000000
    draws_content: 1
    scrollable: 0
    contents_opaque: 1
    transform_tree_index: 2
    clip_tree_index: 1
    effect_tree_index: 1
    scroll_tree_index: 1
  layer id 14
    ...

There is currently no logging for cc::Layer so a selection of important
properties has been dumped.

Bug: 854200
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I938492167619c262976083291bd6db9b4a39fef5
Reviewed-on: https://chromium-review.googlesource.com/1175855
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583755}

cc/trees/layer_tree_host.cc

@@ -795,6 +795,22 @@ bool LayerTreeHost::DoUpdateLayers(Layer* root_layer) {
         std::getline(ss, line);
         VLOG(3) << line;
       }
+
+      VLOG(3) << "CC Layer List:";
+      for (auto* layer : *this) {
+        VLOG(3) << "layer id " << layer->id();
+        VLOG(3) << "  element_id: " << layer->element_id();
+        VLOG(3) << "  bounds: " << layer->bounds().ToString();
+        VLOG(3) << "  opacity: " << layer->opacity();
+        VLOG(3) << "  position: " << layer->position().ToString();
+        VLOG(3) << "  draws_content: " << layer->DrawsContent();
+        VLOG(3) << "  scrollable: " << layer->scrollable();
+        VLOG(3) << "  contents_opaque: " << layer->contents_opaque();
+        VLOG(3) << "  transform_tree_index: " << layer->transform_tree_index();
+        VLOG(3) << "  clip_tree_index: " << layer->clip_tree_index();
+        VLOG(3) << "  effect_tree_index: " << layer->effect_tree_index();
+        VLOG(3) << "  scroll_tree_index: " << layer->scroll_tree_index();
+      }
     }
 
   bool painted_content_has_slow_paths = false;