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

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index 18aaf164228ffe7f528893ee76c72051d3447da0..076ac3be2034a2e90379e32f2df9808cba6f4a65 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -166,6 +166,23 @@ void CancelAllTouches(UIScrollView* web_scroll_view) {
   }
 }
 
+// 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,

[Eugene But (OOO till 7-30), 2016/01/20 16:45:04]

Back usually means previous (not negative delta), also distinguishing between
back/forward/delta in probably unnecessary. With that I think we need 2 metrics:
FAST and SLOW.

[stkhapugin, 2016/01/21 10:39:36]

Stuart mentioned that he'd like to have a distinction between back/forward,
although it's not necessary. Now that I understand how to implement it, why
don't we keep the two? I think there is little reason to distinguish between "go
back one page" and "go back (>1) pages", too. 
Stuart, what do you think? 

[Eugene But (OOO till 7-30), 2016/01/21 15:03:49]

The problem here is that FAST_BACK is misleading, because it also contains
negative delta.

[stuartmorgan, 2016/01/28 23:25:44]

By "negative delta" do you mean going back N>1 pages? I think that's fine
personally, but having either more (1 vs >1 in each direction) or fewer (just
fast or slow history) buckets is fine with me as well.

+  // 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
+};
+
 }  // namespace
 
 @interface CRWWebController () <CRWNativeContentDelegate,
@@ -1686,6 +1703,11 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
   }
 }
 
+- (BOOL)canPerformFastNavigationForSessionEntry:(CRWSessionEntry*)sessionEntry {
+  // Should be overriden by subclasses if they support fast navigation.
+  return NO;
+}
+
 - (void)abortLoad {
   [self abortWebLoad];
   [self loadCancelled];
@@ -1753,6 +1775,26 @@ const NSTimeInterval kSnapshotOverlayTransition = 0.5;
     [self finishHistoryNavigationFromEntry:fromEntry];
     _webStateImpl->SetCacheMode(net::RequestTracker::CACHE_NORMAL);
   }
+
+  // If using WKWebView, report slow/fast navigation metric.
+  if ([self isKindOfClass:[CRWWKWebViewWebController class]]) {

[Eugene But (OOO till 7-30), 2016/01/20 16:45:04]

I think you should move this code to subclass' goDelta: Hardcoding classname is
not maintainable. Also this way you won't need
canPerformFastNavigationForSessionEntry:

[stkhapugin, 2016/01/21 10:39:36]

Done.

+    BOOL isFast =
+        [self canPerformFastNavigationForSessionEntry:self.currentSessionEntry];
+    BOOL isBack = (delta < 0);
+
+    BackForwardNavigationType navigationType;

[Eugene But (OOO till 7-30), 2016/01/20 16:45:04]

Please always initialize local variables.

[stkhapugin, 2016/01/21 10:39:36]

Done.

+    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);
+  }
 }
 
 - (BOOL)isLoaded {