Bug 1367577 - Stylo changes for updated theme constants. r=xidorn

This is a little bit odd, because I didn't want to do all the work to
convert the theme constants to an enum class (although that would be
nice to do in the future).  So this makes them a regular enum, but still
uses gecko_constant_prefix rather than gecko_enum_prefix because that
makes it possible to add the NS_THEME_ prefix that's still present.

MozReview-Commit-ID: LdAyCNCvWT2

--HG--
extra : rebase_source : 9cf0511373f7e9ac51c964cd228776ee17ee5e5e

layout/style/ServoBindings.toml

@@ -86,6 +86,7 @@ bitfield-enums = [
 constified-enums = [
     "UpdateAnimationsTasks",
     "ParsingMode",
+    "ThemeWidgetType",
 ]
 constified-enum-variants = [
     { enum = "nsCSSPropertyID", variants = ["eCSSProperty_COUNT.*"] },
@@ -244,6 +245,7 @@ whitelist-types = [
     "StyleGeometryBox",
     "StyleShapeSource",
     "StyleTransition",
+    "ThemeWidgetType",
     "mozilla::UniquePtr",
     "mozilla::DefaultDelete",
     "mozilla::Side",