CSS: opacity does not accept keywords

The Typed OM types for opacity should not include Keyword.
https://drafts.csswg.org/css-color/#transparency

Change-Id: I6a476f29fee622d962483aeabe40ff2e1acfb3f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1734757
Commit-Queue: Darren Shen <shend@chromium.org>
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683894}

third_party/blink/renderer/core/css/css_properties.json5

@@ -2375,7 +2375,7 @@
       default_value: "1.0",
       type_name: "float",
       computed_style_custom_functions: ["setter"],
-      typedom_types: ["Keyword", "Number"]
+      typedom_types: ["Number"]
     },
     {
       name: "order",