Explicitly disable UKM reporting for MQ evaluation.

This is in preparation for avoiding excessive UKM reporting for viewport
size media queries in [1].

The skip_ukm_reporting_ will be set to true when we detect if any media
queries have changed, e.g. on window resize. The new result will be
detected later in RuleSet::AddRulesFromSheet for stylesheets and
MediaQueryList::UpdateMatches() for matchMedia() when the evaluation
actually changed.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2449913

Bug: 973801
Change-Id: Ic8db810c8be2f223e08113db040d5515c42e6592
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2479372Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818030}

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

@@ -159,6 +159,7 @@ bool MediaQueryEvaluator::Eval(
 
 bool MediaQueryEvaluator::DidResultsChange(
     const MediaQueryResultList& results) const {
+  base::AutoReset<bool> skip(&skip_ukm_reporting_, true);
   for (auto& result : results) {
     if (Eval(result.Expression()) != result.Result())
       return true;
@@ -166,6 +167,16 @@ bool MediaQueryEvaluator::DidResultsChange(
   return false;
 }
 
+bool MediaQueryEvaluator::DidResultsChange(
+    const Vector<MediaQuerySetResult>& results) const {
+  base::AutoReset<bool> skip(&skip_ukm_reporting_, true);
+  for (const auto& result : results) {
+    if (result.Result() != Eval(result.MediaQueries()))
+      return true;
+  }
+  return false;
+}
+
 template <typename T>
 bool CompareValue(T a, T b, MediaFeaturePrefix op) {
   switch (op) {

third_party/blink/renderer/core/css/media_query_evaluator.h

@@ -38,6 +38,7 @@ class MediaQuery;
 class MediaQueryExp;
 class MediaQueryResult;
 class MediaQuerySet;
+class MediaQuerySetResult;
 class MediaValues;
 class MediaValuesInitialViewport;
 
@@ -98,6 +99,10 @@ class CORE_EXPORT MediaQueryEvaluator final
   // evaluation.
   bool DidResultsChange(const MediaQueryResultList& results) const;
 
+  // Returns true if any of the media queries in the results lists changed its
+  // evaluation.
+  bool DidResultsChange(const Vector<MediaQuerySetResult>& results) const;
+
   void Trace(Visitor*) const;
 
  private:
@@ -105,6 +110,10 @@ class CORE_EXPORT MediaQueryEvaluator final
 
   String media_type_;
   Member<MediaValues> media_values_;
+
+  // Even if UKM reporting is enabled, do not report any media query evaluation
+  // results if this is set to true.
+  mutable bool skip_ukm_reporting_{false};
 };
 
 }  // namespace blink

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

@@ -431,11 +431,7 @@ void RuleSet::CompactRules() {
 
 bool RuleSet::DidMediaQueryResultsChange(
     const MediaQueryEvaluator& evaluator) const {
-  for (const auto& result : media_query_set_results_) {
-    if (result.Result() != evaluator.Eval(result.MediaQueries()))
-      return true;
-  }
-  return false;
+  return evaluator.DidResultsChange(media_query_set_results_);
 }
 
 void MinimalRuleData::Trace(Visitor* visitor) const {