Add a generated ComputedStyleBase class that ComputedStyle extends

third_party/WebKit/Source/build/scripts/templates/BaseComputedStyleConstants.h.tmpl

Index: third_party/WebKit/Source/build/scripts/templates/BaseComputedStyleConstants.h.tmpl
diff --git a/third_party/WebKit/Source/build/scripts/templates/BaseComputedStyleConstants.h.tmpl b/third_party/WebKit/Source/build/scripts/templates/BaseComputedStyleConstants.h.tmpl
new file mode 100644
index 0000000000000000000000000000000000000000..e3dfabd06a77551667dd0c4e98e3d8f1f3803042
--- /dev/null
+++ b/third_party/WebKit/Source/build/scripts/templates/BaseComputedStyleConstants.h.tmpl
@@ -0,0 +1,25 @@
+{% from 'macros.tmpl' import license %}
+{{license()}}
+
+#ifndef BaseComputedStyleConstants_h
+#define BaseComputedStyleConstants_h
+
+namespace blink {
+
+{% for enum_name, enum_values in enums.items() %}
+enum class {{enum_name}} {
+    {% for enum_value in enum_values %}
+    {{enum_value}},
+    {% endfor %}
+};
+
+// TODO(sashab): Remove this and update callsites to use a boolean.
+enum IsAtShadowBoundary {
+    AtShadowBoundary,
+    NotAtShadowBoundary,
+};
+
+{% endfor %}
+} // namespace blink
+
+#endif // BaseComputedStyleConstants_h