Made property class headers more readable by adding comment

This is to prevent upcoming larger style builder functions from making
the headers difficult to read

This patch
- Added a comment to section off style builder functions in the property
  class header files for readability
- Moved style builder functions to end of class so that that section
  does not merge with another optional section
- Changed the condition to print style builder functions from
  should_declare to should_implement so that the comment does not appear
  for classes that have not yet had the style builder methods moved into
  them. The should_declare member is kept as it will be required in
  future for declaration only cases.

Diff of generated files:
https://gist.github.com/BugsNash/49febc392af82920fea3e59e6c7d3224/revisions

Bug: 751354
Change-Id: Ic664b6fcad97d70bf24d46428561fff2dd6107a0
Reviewed-on: https://chromium-review.googlesource.com/780160
Commit-Queue: Bugs Nash <bugsnash@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518372}

third_party/WebKit/Source/build/scripts/core/css/properties/templates/CSSPropertySubclass.h.tmpl

@@ -57,9 +57,6 @@ class {{property_classname}} : public {{property.superclass}} {
   {% if property.is_descriptor %}
   bool IsDescriptor() const override { return true; }
   {% endif %}
-  {% if property.should_declare_apply_functions %}
-{{style_builder_functions(property)}}
-  {%- endif %}
   {% if 'Percent' in property.typedom_types %}
   bool SupportsPercentage() const override { return true; }
   {% endif %}
@@ -104,6 +101,11 @@ class {{property_classname}} : public {{property.superclass}} {
   }
     {% endif %}
   {% endif %}
+  {% if property.should_implement_apply_functions %}
+
+  // Style builder functions
+{{style_builder_functions(property)}}
+  {%- endif %}
 };
 
 }  // namespace CSS{{property.superclass}}