Add counter for number of page loads where doc.write blocking triggers.

chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
index 0261f1d36c19094c991459707331ad842d3b2618..a4358a64dc36a5b68cd4718045b54dcf2f914ffa 100644
--- a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
@@ -13,6 +13,7 @@ namespace internal {
 // Expose metrics for tests.
 extern const char kHistogramDocWriteParseStartToFirstContentfulPaint[];
 extern const char kHistogramDocWriteBlockParseStartToFirstContentfulPaint[];
+extern const char kHistogramDocWriteBlockCount[];
 extern const char kHistogramDocWriteBlockReloadCount[];
 
 }  // namespace internal
@@ -20,7 +21,8 @@ extern const char kHistogramDocWriteBlockReloadCount[];
 class DocumentWritePageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
-  DocumentWritePageLoadMetricsObserver();
+  DocumentWritePageLoadMetricsObserver() = default;
+
   // page_load_metrics::PageLoadMetricsObserver implementation:
   void OnFirstContentfulPaint(
       const page_load_metrics::PageLoadTiming& timing,
@@ -62,7 +64,8 @@ class DocumentWritePageLoadMetricsObserver
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& info);
 
-  bool doc_write_block_reload_observed_;
+  bool doc_write_block_observed_ = false;
+  bool doc_write_block_reload_observed_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(DocumentWritePageLoadMetricsObserver);
 };