css: fix an instance of bugprone-unused-raii

It seems that this constructor call was intended to be used as a scope
guard. In order for that to happen, there has to be a variable name
here.

Bug: 1130379
Change-Id: I37569794b4526ab0c90e82b0a133ca1d85bb14b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2420917Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808870}

third_party/blink/renderer/core/css/css_keyframe_rule.cc

@@ -44,7 +44,7 @@ CSSKeyframeRule::~CSSKeyframeRule() = default;
 
 void CSSKeyframeRule::setKeyText(const String& key_text,
                                  ExceptionState& exception_state) {
-  CSSStyleSheet::RuleMutationScope(this);
+  CSSStyleSheet::RuleMutationScope rule_mutation_scope(this);
 
   if (!keyframe_->SetKeyText(key_text))
     exception_state.ThrowDOMException(