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

chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
index dbb8ac05af090d9e729de2e8a1d8634c5095f1d3..2c543d1e587876b547ccef17c03a7f2a2947ce03 100644
--- a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
@@ -67,14 +67,13 @@ const char kBackgroundDocWriteBlockParseBlockedOnScriptLoadDocumentWrite[] =
 const char kBackgroundHistogramDocWriteBlockParseDuration[] =
     "PageLoad.Clients.DocWrite.Block.ParseTiming.ParseDuration.Background";
 
+const char kHistogramDocWriteBlockCount[] =
+    "PageLoad.Clients.DocWrite.Block.Count";
 const char kHistogramDocWriteBlockReloadCount[] =
     "PageLoad.Clients.DocWrite.Block.ReloadCount";
 
 }  // namespace internal
 
-DocumentWritePageLoadMetricsObserver::DocumentWritePageLoadMetricsObserver()
-    : doc_write_block_reload_observed_(false) {}
-
 void DocumentWritePageLoadMetricsObserver::OnFirstContentfulPaint(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {
@@ -126,6 +125,12 @@ void DocumentWritePageLoadMetricsObserver::OnLoadingBehaviorObserved(
     UMA_HISTOGRAM_COUNTS(internal::kHistogramDocWriteBlockReloadCount, 1);
     doc_write_block_reload_observed_ = true;
   }
+  if ((info.metadata.behavior_flags &
+       blink::WebLoadingBehaviorFlag::WebLoadingBehaviorDocumentWriteBlock) &&
+      !doc_write_block_observed_) {
+    UMA_HISTOGRAM_BOOLEAN(internal::kHistogramDocWriteBlockCount, true);
+    doc_write_block_observed_ = true;
+  }
 }
 
 void DocumentWritePageLoadMetricsObserver::