[LazyParseCSS] Add histogram for rule usage %

third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp

Index: third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
diff --git a/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp b/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
index 8370f0a0dbd447696b93fc7dd2c55b6cb6698e6b..1a231c6dc85ded6e06f0492cf21242bf60b8b075 100644
--- a/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
+++ b/third_party/WebKit/Source/core/css/parser/CSSLazyParsingState.cpp
@@ -3,8 +3,10 @@
 // found in the LICENSE file.
 
 #include "core/css/parser/CSSLazyParsingState.h"
+#include "core/css/parser/CSSLazyPropertyParserImpl.h"
 #include "core/css/parser/CSSParserTokenRange.h"
 #include "core/frame/UseCounter.h"
+#include "platform/Histogram.h"
 
 namespace blink {
 
@@ -15,7 +17,19 @@ CSSLazyParsingState::CSSLazyParsingState(const CSSParserContext& context,
     : m_context(context),
       m_escapedStrings(std::move(escapedStrings)),
       m_sheetText(sheetText),
-      m_owningContents(contents) {}
+      m_owningContents(contents),
+      m_parsedStyleRules(0),
+      m_totalStyleRules(0),
+      m_styleRulesNeededForNextMilestone(0),
+      m_usage(UsageGe0) {
+  recordUsageMetrics();
+}
+
+CSSLazyPropertyParserImpl* CSSLazyParsingState::createLazyParser(
+    const CSSParserTokenRange& block) {
+  ++m_totalStyleRules;
+  return new CSSLazyPropertyParserImpl(std::move(block), this);
+}
 
 const CSSParserContext& CSSLazyParsingState::context() {
   DCHECK(m_owningContents);
@@ -25,9 +39,18 @@ const CSSParserContext& CSSLazyParsingState::context() {
   return m_context;
 }
 
+void CSSLazyParsingState::countRuleParsed() {
+  ++m_parsedStyleRules;
+  while (m_parsedStyleRules > m_styleRulesNeededForNextMilestone) {
+    DCHECK_NE(UsageAll, m_usage);
+    ++m_usage;
+    recordUsageMetrics();
+  }
+}
+
 bool CSSLazyParsingState::shouldLazilyParseProperties(
     const CSSSelectorList& selectors,
-    const CSSParserTokenRange& block) {
+    const CSSParserTokenRange& block) const {
   // Simple heuristic for an empty block. Note that |block| here does not
   // include {} brackets. We avoid lazy parsing empty blocks so we can avoid
   // considering them when possible for matching. Lazy blocks must always be
@@ -53,4 +76,34 @@ bool CSSLazyParsingState::shouldLazilyParseProperties(
   return true;
 }
 
+void CSSLazyParsingState::recordUsageMetrics() {
+  DEFINE_STATIC_LOCAL(EnumerationHistogram, usageHistogram,
+                      ("Style.LazyUsage.Percent", UsageLastValue));
+  switch (m_usage) {
+    case UsageGe0:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules * .1;
+      break;
+    case UsageGt10:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules * .25;
+      break;
+    case UsageGt25:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules * .5;
+      break;
+    case UsageGt50:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules * .75;
+      break;
+    case UsageGt75:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules * .9;
+      break;
+    case UsageGt90:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules - 1;
+      break;
+    case UsageAll:
+      m_styleRulesNeededForNextMilestone = m_totalStyleRules;
+      break;
+  }
+
+  usageHistogram.count(m_usage);
+}
+
 }  // namespace blink