Fail earlier when parsing "@scroll-timeline" with flag disabled

Before we implemented @scroll-timeline, the parser would convert a
"@scroll-timeline" into CSSAtRuleInvalid, and then go through the
ConsumeErroneousAtRule() code path. Now even with the flag disabled, we
still parse "@scroll-timeline" into CSSAtRuleScrollTimeline and then go
through the ConsumeScrollTimeline() code path and fail inside.

This patch parses "@scroll-timeline" into CSSAtRuleInvalid() so that the
when the flag is disabled, parser behavior is exactly the same as if we
haven't implemented it at all.

Change-Id: Ib83bbc19fe2715f52d10a9ce5d351670dcb8bd15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522050
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824640}

third_party/blink/renderer/core/css/parser/css_at_rule_id.cc

@@ -32,8 +32,11 @@ CSSAtRuleID CssAtRuleID(StringView name) {
       return kCSSAtRuleCounterStyle;
     return kCSSAtRuleInvalid;
   }
-  if (EqualIgnoringASCIICase(name, "scroll-timeline"))
-    return kCSSAtRuleScrollTimeline;
+  if (EqualIgnoringASCIICase(name, "scroll-timeline")) {
+    if (RuntimeEnabledFeatures::CSSScrollTimelineEnabled())
+      return kCSSAtRuleScrollTimeline;
+    return kCSSAtRuleInvalid;
+  }
   if (EqualIgnoringASCIICase(name, "supports"))
     return kCSSAtRuleSupports;
   if (EqualIgnoringASCIICase(name, "viewport"))