Add UMA to count the number of Google CAPTCHA pages shown and solved by users.

components/page_load_metrics/browser/metrics_web_contents_observer.h

Index: components/page_load_metrics/browser/metrics_web_contents_observer.h
diff --git a/components/page_load_metrics/browser/metrics_web_contents_observer.h b/components/page_load_metrics/browser/metrics_web_contents_observer.h
index 4a9e061a00a94cdb55701d37ebe20d12e894aca1..95883df7b5fe669a007e3e129610cf4a81746594 100644
--- a/components/page_load_metrics/browser/metrics_web_contents_observer.h
+++ b/components/page_load_metrics/browser/metrics_web_contents_observer.h
@@ -33,6 +33,9 @@ class RapporService;
 
 namespace page_load_metrics {
 
+// Returns true if the given URL matches a Google CAPTCHA page.
+bool IsGoogleCaptcha(const GURL& url);
+
 // These constants are for keeping the tests in sync.
 const char kHistogramNameFirstLayout[] =
     "PageLoad.Timing2.NavigationToFirstLayout";
@@ -239,6 +242,8 @@ class MetricsWebContentsObserver
       content::NavigationHandle* navigation_handle) override;
   void DidFinishNavigation(
       content::NavigationHandle* navigation_handle) override;
+  void DidRedirectNavigation(
+      content::NavigationHandle* navigation_handle) override;
 
   void WasShown() override;
   void WasHidden() override;