Add CSSTiming to collect aggregate PLT-level stats about CSS.

chrome/common/page_load_metrics/page_load_timing.cc

Index: chrome/common/page_load_metrics/page_load_timing.cc
diff --git a/chrome/common/page_load_metrics/page_load_timing.cc b/chrome/common/page_load_metrics/page_load_timing.cc
index 3f0925893c40a0eba879b36cdb7bafa3130c783c..40120a3a406d14b5696d9dfc4bed2026db9de77b 100644
--- a/chrome/common/page_load_metrics/page_load_timing.cc
+++ b/chrome/common/page_load_metrics/page_load_timing.cc
@@ -6,6 +6,21 @@
 
 namespace page_load_metrics {
 
+StyleSheetTiming::StyleSheetTiming() {}
+
+StyleSheetTiming::StyleSheetTiming(const StyleSheetTiming& other) = default;
+
+StyleSheetTiming::~StyleSheetTiming() {}
+
+bool StyleSheetTiming::operator==(const StyleSheetTiming& other) const {
+  return author_style_sheet_parse_duration_before_fcp ==
+         other.author_style_sheet_parse_duration_before_fcp;
+}
+
+bool StyleSheetTiming::IsEmpty() const {
+  return !author_style_sheet_parse_duration_before_fcp;
+}
+
 PageLoadTiming::PageLoadTiming() {}
 
 PageLoadTiming::PageLoadTiming(const PageLoadTiming& other) = default;
@@ -33,7 +48,8 @@ bool PageLoadTiming::operator==(const PageLoadTiming& other) const {
              other.parse_blocked_on_script_execution_duration &&
          parse_blocked_on_script_execution_from_document_write_duration ==
              other
-                 .parse_blocked_on_script_execution_from_document_write_duration;
+                 .parse_blocked_on_script_execution_from_document_write_duration &&
+         style_sheet_timing == other.style_sheet_timing;
 }
 
 bool PageLoadTiming::IsEmpty() const {
@@ -45,7 +61,8 @@ bool PageLoadTiming::IsEmpty() const {
          !parse_blocked_on_script_load_duration &&
          !parse_blocked_on_script_load_from_document_write_duration &&
          !parse_blocked_on_script_execution_duration &&
-         !parse_blocked_on_script_execution_from_document_write_duration;
+         !parse_blocked_on_script_execution_from_document_write_duration &&
+         style_sheet_timing.IsEmpty();
 }
 
 PageLoadMetadata::PageLoadMetadata() {}