iOS: Add performance metric for context menu element fetch duration.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ia4cdf1f83336fc4f1c5b8dff8891e323ad4c0a14
Reviewed-on: https://chromium-review.googlesource.com/883042
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534201}

ios/web/web_state/ui/crw_context_menu_controller.mm

@@ -75,6 +75,9 @@ void CancelTouches(UIGestureRecognizer* gesture_recognizer) {
              completionHandler:(void (^)(NSDictionary*))handler;
 // Sets the value of |_DOMElementForLastTouch|.
 - (void)setDOMElementForLastTouch:(NSDictionary*)element;
+// Logs the time taken to fetch DOM element details.
+- (void)logElementFetchDurationWithStartTime:
+    (base::TimeTicks)elementFetchStartTime;
 // Forwards the execution of |script| to |javaScriptDelegate| and if it is nil,
 // to |webView|.
 - (void)executeJavaScript:(NSString*)script
@@ -286,6 +289,12 @@ void CancelTouches(UIGestureRecognizer* gesture_recognizer) {
   }
 }
 
+- (void)logElementFetchDurationWithStartTime:
+    (base::TimeTicks)elementFetchStartTime {
+  UMA_HISTOGRAM_TIMES("ContextMenu.DOMElementFetchDuration",
+                      base::TimeTicks::Now() - elementFetchStartTime);
+}
+
 #pragma mark -
 #pragma mark UIGestureRecognizerDelegate
 
@@ -309,10 +318,14 @@ void CancelTouches(UIGestureRecognizer* gesture_recognizer) {
   // fetched - system context menu will be shown.
   [self setDOMElementForLastTouch:nil];
   _contextMenuNeedsDisplay = NO;
+
+  base::TimeTicks fetchStartTime = base::TimeTicks::Now();
   __weak CRWContextMenuController* weakSelf = self;
   [self fetchDOMElementAtPoint:[touch locationInView:_webView]
              completionHandler:^(NSDictionary* element) {
-               [weakSelf setDOMElementForLastTouch:element];
+               CRWContextMenuController* strongSelf = weakSelf;
+               [strongSelf logElementFetchDurationWithStartTime:fetchStartTime];
+               [strongSelf setDOMElementForLastTouch:element];
              }];
   return YES;
 }

tools/metrics/histograms/histograms.xml

@@ -11716,6 +11716,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ContextMenu.DOMElementFetchDuration" units="ms">
+  <owner>michaeldo@chromium.org</owner>
+  <summary>
+    How long it takes to fetch the details of the DOM element the user is
+    selecting for the context menu. On iOS, JavaScript must be run in order to
+    get the details of the DOM element. If the element is not inside the main
+    frame, latency could be added due to the communication between frames. Only
+    logged on iOS.
+  </summary>
+</histogram>
+
 <histogram name="ContextMenu.SaveLinkType" enum="ContextMenuSaveLinkType">
   <owner>qinmin@chromium.org</owner>
   <summary>