Add Layer Id information to the LayerTree json string

LayerTreeAsJson is useful during debugging of layer tree operations.
Adding the layer id into the information for the layer currently being
serialized. This helps the function be more usable as user knows which
layer he is looking when checking the json.

BUG=None

R=danakj,enne

Review URL: https://codereview.chromium.org/1140913002

Cr-Commit-Position: refs/heads/master@{#330306}

cc/layers/layer_impl.cc

@@ -656,6 +656,7 @@ gfx::Vector2dF LayerImpl::FixedContainerSizeDelta() const {
 
 base::DictionaryValue* LayerImpl::LayerTreeAsJson() const {
   base::DictionaryValue* result = new base::DictionaryValue;
+  result->SetInteger("LayerId", id());
   result->SetString("LayerType", LayerTypeAsString());
 
   base::ListValue* list = new base::ListValue;