Separate CSSValueID mappings generator from ComputedStyleBase generator.

third_party/WebKit/Source/core/BUILD.gn

Index: third_party/WebKit/Source/core/BUILD.gn
diff --git a/third_party/WebKit/Source/core/BUILD.gn b/third_party/WebKit/Source/core/BUILD.gn
index 4e8b448aaa51077524a034f1ef9570534a4e76e9..1a8a4281d4490d31da890ecd06e734937b1b28f7 100644
--- a/third_party/WebKit/Source/core/BUILD.gn
+++ b/third_party/WebKit/Source/core/BUILD.gn
@@ -418,12 +418,19 @@ css_properties("make_core_generated_computed_style_base") {
     "../build/scripts/templates/ComputedStyleBase.cpp.tmpl",
     "../build/scripts/templates/ComputedStyleBase.h.tmpl",
     "../build/scripts/templates/ComputedStyleBaseConstants.h.tmpl",
-    "../build/scripts/templates/CSSValueIDMappingsGenerated.h.tmpl",
   ]
   outputs = [
     "$blink_core_output_dir/ComputedStyleBase.cpp",
     "$blink_core_output_dir/ComputedStyleBase.h",
     "$blink_core_output_dir/ComputedStyleBaseConstants.h",
+  ]
+}
+
+css_properties("make_core_generated_css_value_id_mappings") {
+  script = "../build/scripts/make_css_value_id_mappings.py"
+  other_inputs =
+      [ "../build/scripts/templates/CSSValueIDMappingsGenerated.h.tmpl" ]
+  outputs = [
     "$blink_core_output_dir/CSSValueIDMappingsGenerated.h",
   ]
 }