[page_load_metrics] Add support for chrome:// urls and add extensions observer

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
index 6077b6a2abcbb8bbb72cb6d448b736bac50c8b94..888ecd4f2ce91f4372c4e80017e3dd3321901896 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
@@ -10,6 +10,7 @@
 #include "chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h"
+#include "chrome/browser/page_load_metrics/observers/webui_page_load_metrics_observer.h"
 #include "chrome/browser/prerender/prerender_histograms.h"
 #include "chrome/browser/prerender/prerender_origin.h"
 #include "chrome/browser/profiles/profile.h"
@@ -280,6 +281,22 @@ IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, NoDocumentWriteScript) {
       internal::kHistogramDocWriteBlockParseStartToFirstContentfulPaint, 0);
 }
 
+IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, ExtensionsPage) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+  ui_test_utils::NavigateToURL(browser(), GURL("chrome://extensions"));
+  NavigateToUntrackedUrl();
+  histogram_tester_.ExpectTotalCount(
+      internal::kHistogramExtensionsParseDuration, 1);
+}
+
+IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, ExtensionsPageAbout) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+  ui_test_utils::NavigateToURL(browser(), GURL("about:extensions"));
+  NavigateToUntrackedUrl();
+  histogram_tester_.ExpectTotalCount(
+      internal::kHistogramExtensionsParseDuration, 1);
+}
+
 IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest, BadXhtml) {
   ASSERT_TRUE(embedded_test_server()->Start());