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

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index fb23e7c6c99fea41c095ec37de390db4a1d34c4a..aeb1001c336caa3ace2555cc6d6016a575f4e0a8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -21622,6 +21622,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Navigation.IOSWKWebViewSlowFastBackForward"
+    enum="BackForwardNavigationType">
+  <owner>stkhapugin@chromium.org</owner>
+  <owner>eugenebut@chromium.org</owner>
+  <owner>stuartmorgan@chromium.org</owner>
+  <summary>
+    Counts slow/fast back/forward navigations on iOS with WKWebView enabled.
+    Fast navigation is a back/forward navigation done with WKBackForwardList.
+  </summary>
+</histogram>
+
 <histogram name="Navigation.IsMobileOptimized" enum="BooleanIsMobileOptimized">
   <owner>cjhopman@chromium.org</owner>
   <owner>nyquist@chromium.org</owner>
@@ -57045,6 +57056,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="3" label="[Deprecated] AUTO_LAUNCH_FOREGROUND_USELESS"/>
 </enum>
 
+<enum name="BackForwardNavigationType" type="int">
+  <int value="0" label="Fast back navigation with WKBackForwardList"/>
+  <int value="1" label="Slow back navigation"/>
+  <int value="2" label="Fast forward navigation with WKBackForwardList"/>
+  <int value="3" label="Slow forward navigation"/>
+</enum>
+
 <enum name="BackgroundModeMenuItem" type="int">
   <int value="0" label="About"/>
   <int value="1" label="Task manager"/>