Added property id comments to CSSDescriptorIndices entries.

Added trailing comments displaying the property api pointed to by all
the non-null entries of the CSSDescriptorIndices table in
CSSPropertyDescriptor.cpp. This improves the readability of the table.

Changes to generated files:
https://gist.github.com/BugsNash/44fa796f876d5b4727931a4b63cec386/revisions

BUG=668012

Review-Url: https://codereview.chromium.org/2849133002
Cr-Commit-Position: refs/heads/master@{#468490}

third_party/WebKit/Source/build/scripts/make_css_property_apis.py

@@ -94,11 +94,11 @@ class CSSPropertyAPIWriter(StyleBuilderWriter):
         self._invalid_descriptor_index = 0
         # Initialize the whole thing to the invalid descriptor to handle gaps
         num_indices = self.last_unresolved_property_id + 1
-        self._descriptor_indices = [self._invalid_descriptor_index] * num_indices
+        self._descriptor_indices = dict.fromkeys(xrange(num_indices), {'id': self._invalid_descriptor_index, 'api': None})
         # Now populate all entries for which there exists a class, i.e. that aren't gaps
         for api_class in self._api_classes:
             for property_enum in property_enums_for_class[api_class.classname]:
-                self._descriptor_indices[property_enum] = api_class.index
+                self._descriptor_indices[property_enum] = {'id': api_class.index, 'api': api_class.classname}
 
     @template_expander.use_jinja('CSSPropertyDescriptor.cpp.tmpl')
     def generate_property_descriptor_cpp(self):

third_party/WebKit/Source/build/scripts/templates/CSSPropertyDescriptor.cpp.tmpl

+{% from 'macros.tmpl' import license, print_if %}
 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
@@ -45,8 +46,8 @@ static_assert(cssPropertyDescriptors[INVALID_DESCRIPTOR_INDEX].{{api_method_name
 // Lookup table mapping CSSPropertyID to index in the cssPropertyDescriptors
 // table
 static size_t CSSDescriptorIndices[] = {
-   {% for id in descriptor_indices %}
-   {{id}},
+   {% for item in descriptor_indices %}
+   {{descriptor_indices[item].id}},{{print_if(descriptor_indices[item].api != None, ' // ' ~ descriptor_indices[item].api)}}
    {% endfor %}
 };