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 80cfbc7de7b75335eeb51c86416f5d71150721f6..5c6437f0f91783acdef04732ecb5f5ccf164af86 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22036,6 +22036,17 @@ 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>
@@ -57963,6 +57974,13 @@ 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"/>