Adds UMA for slow and fast back/forward WKWebController navigations.

ios/web/web_state/ui/crw_wk_web_view_web_controller.mm

Index: ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
index de29b293c2da896d3da9b639632678324333a1ba..e2d18a492604db1f08047168fd73660d2b4d10f4 100644
--- a/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
+++ b/ios/web/web_state/ui/crw_wk_web_view_web_controller.mm
@@ -89,6 +89,23 @@ NSString* GetRefererFromNavigationAction(WKNavigationAction* action) {
 NSString* const kScriptMessageName = @"crwebinvoke";
 NSString* const kScriptImmediateName = @"crwebinvokeimmediate";
 
+// Key of the UMA Navigation.IOSWKWebViewSlowFastBackForward histogram.
+const char kUMAWKWebViewSlowFastBackForwardNavigationKey[] =
+    "Navigation.IOSWKWebViewSlowFastBackForward";
+
+// Values for the histogram that counts slow/fast back/forward navigations.
+enum BackForwardNavigationType {
+  // Fast back navigation through WKWebView back-forward list.
+  FAST_BACK = 0,
+  // Slow back navigation when back-forward list navigation is not possible.
+  SLOW_BACK,
+  // Fast forward navigation through WKWebView back-forward list.
+  FAST_FORWARD,
+  // Slow forward navigation when back-forward list navigation is not possible.
+  SLOW_FORWARD,
+  BACK_FORWARD_NAVIGATION_TYPE_COUNT
+};
+
 // Utility functions for storing the source of NSErrors received by WKWebViews:
 // - Errors received by |-webView:didFailProvisionalNavigation:withError:| are
 //   recorded using WKWebViewErrorSource::PROVISIONAL_LOAD.  These should be
@@ -298,6 +315,12 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
 // navigation.
 - (BOOL)isBackForwardListItemValid:(WKBackForwardListItem*)item;
 
+// Indicates if navigating using WKBackForwardList API is possible.
+// Returns YES if there is a corresponding WKBackForwardListItem and it is in
+// the current WKWebView's back-forward list.
+- (BOOL)isNavigationPerformedWithBackForwardList:
+    (web::WKBackForwardListItemHolder*)holder;
+
 // Called when web view process has been terminated.
 - (void)webViewWebProcessDidCrash;
 
@@ -473,6 +496,33 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
   [super stopLoading];
 }
 
+- (void)goDelta:(int)delta {
+  [super goDelta:delta];
+  if (delta == 0)
+    return;
+
+  // Report slow/fast navigation metric.
+  web::WKBackForwardListItemHolder* holder =
+      web::WKBackForwardListItemHolder::FromNavigationItem(
+          self.currentSessionEntry.navigationItemImpl);
+  BOOL isFast = [self isNavigationPerformedWithBackForwardList:holder];
+  BOOL isBack = (delta < 0);
+
+  BackForwardNavigationType navigationType =
+      BackForwardNavigationType::FAST_BACK;
+  if (isBack) {
+    navigationType = isFast ? BackForwardNavigationType::FAST_BACK
+                            : BackForwardNavigationType::SLOW_BACK;
+  } else {
+    navigationType = isFast ? BackForwardNavigationType::FAST_FORWARD
+                            : BackForwardNavigationType::SLOW_FORWARD;
+  }
+
+  UMA_HISTOGRAM_ENUMERATION(
+      kUMAWKWebViewSlowFastBackForwardNavigationKey, navigationType,
+      BackForwardNavigationType::BACK_FORWARD_NAVIGATION_TYPE_COUNT);
+}
+
 #pragma mark -
 #pragma mark Testing-Only Methods
 
@@ -619,12 +669,9 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
     [self loadRequest:request];
   };
 
-  // If there is no corresponding WKBackForwardListItem, or the item is not in
-  // the current WKWebView's back-forward list, navigating using WKWebView API
-  // is not possible. In this case, fall back to the default navigation
-  // mechanism.
-  if (!holder->back_forward_list_item() ||
-      ![self isBackForwardListItemValid:holder->back_forward_list_item()]) {
+  // If the fast navigation with WKBackForwardList is not possible, fall back to
+  // the default navigation mechanism.
+  if ([self isNavigationPerformedWithBackForwardList:holder]) {
     defaultNavigationBlock();
     return;
   }
@@ -948,6 +995,12 @@ WKWebViewErrorSource WKWebViewErrorSourceFromError(NSError* error) {
          [list.backList indexOfObject:item] != NSNotFound;
 }
 
+- (BOOL)isNavigationPerformedWithBackForwardList:
+    (web::WKBackForwardListItemHolder*)holder {
+  return (holder->back_forward_list_item() &&
+          [self isBackForwardListItemValid:holder->back_forward_list_item()]);
+}
+
 - (void)webViewWebProcessDidCrash {
   _webProcessIsDead = YES;