Fix old links in css property template

Change src/third_party/WebKit/Source/core/css/properties/README.md
to src/third_party/blink/renderer/core/css/properties/README.md

Bug: N/A
Change-Id: I604bc885c459c14ea25d8973de75cf8c04080bec
Reviewed-on: https://chromium-review.googlesource.com/c/1333095Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Amos Lim <eui-sang.lim@samsung.com>
Cr-Commit-Position: refs/heads/master@{#607552}

third_party/blink/renderer/build/scripts/core/css/properties/templates/css_property_subclass.h.tmpl

@@ -21,7 +21,7 @@ namespace CSS{{property.namespace_group}} {
 {% else %}
 // Implements the '{{property.name}}' CSS descriptor
 {% endif %}
-// See src/third_party/WebKit/Source/core/css/properties/README.md
+// See src/third_party/blink/renderer/core/css/properties/README.md
   {% if property.alias_for != None%}
 class {{property_classname}} final : public CSSUnresolvedProperty {
  public: