Support declaring keyword property Gecko coverage as inexhaustive.

This allows us to control whether the catchall case in the match expression
that maps Gecko const to Servo enum will be generated.

MozReview-Commit-ID: L65IjTONdfl

components/style/properties/helpers.mako.rs

@@ -329,7 +329,7 @@
         keyword_kwargs = {a: kwargs.pop(a, None) for a in [
             'gecko_constant_prefix', 'gecko_enum_prefix',
             'extra_gecko_values', 'extra_servo_values',
-            'custom_consts',
+            'custom_consts', 'gecko_inexhaustive',
         ]}
     %>