Perf metric tracking the wallclock time it takes to show a new tab page.

ios/chrome/browser/ui/browser_view_controller.mm

Index: ios/chrome/browser/ui/browser_view_controller.mm
diff --git a/ios/chrome/browser/ui/browser_view_controller.mm b/ios/chrome/browser/ui/browser_view_controller.mm
index 1271cea016bb063baa8cc2dd4162d3f0fff7faca..a3d5756082c305ef847245154fee83e90610ba7d 100644
--- a/ios/chrome/browser/ui/browser_view_controller.mm
+++ b/ios/chrome/browser/ui/browser_view_controller.mm
@@ -572,6 +572,10 @@ NSString* const kNativeControllerTemporaryKey = @"NativeControllerTemporaryKey";
 // the user from interacting with the browser view.
 @property(nonatomic, strong)
     ActivityOverlayCoordinator* activityOverlayCoordinator;
+// A block to be run when the |tabWasAdded:| method completes the animation
+// for the presentation of a new tab. Can be used to record performance metrics.
+@property(nonatomic, strong, nullable)
+    ProceduralBlock tabWasAddedCompletionBlock;
 
 // The user agent type used to load the currently visible page. User agent type
 // is NONE if there is no visible page or visible page is a native page.
@@ -932,6 +936,7 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
 @synthesize hideStatusBar = _hideStatusBar;
 @synthesize activityOverlayCoordinator = _activityOverlayCoordinator;
 @synthesize presenting = _presenting;
+@synthesize tabWasAddedCompletionBlock = _tabWasAddedCompletionBlock;
 
 #pragma mark - Object lifecycle
 
@@ -1198,6 +1203,21 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
 }
 
 - (void)newTab:(id)sender {
+  // Observe the timing of the new tab creation, both MainController
+  // and BrowserViewController call into this method on the correct BVC to
+  // create new tabs making it preferable to doing this in
+  // |chromeExecuteCommand:|.
+  NSTimeInterval startTime = [NSDate timeIntervalSinceReferenceDate];
+  __weak __typeof(self) weakSelf = self;

[rohitrao (ping after 24h), 2017/05/22 12:55:05]

Could we capture the value of isOffTheRecord instead of capturing self?

[PL, 2017/05/23 22:37:55]

Absolutely! Done!

+  self.tabWasAddedCompletionBlock = ^{
+    UMA_HISTOGRAM_TIMES(
+        weakSelf.isOffTheRecord
+            ? "Toolbar.Menu.NewIncognitoTabPresentationDuration"
+            : "Toolbar.Menu.NewTabPresentationDuration",

[Ilya Sherman, 2017/05/22 20:45:45]

Names passed to UMA_HISTOGRAM macros must be runtime constants.  Please either
move the if-stmt out of the macro, or use the non-macro version,
base::UmaHistogramTimes().

[PL, 2017/05/23 22:37:55]

Ah thanks! I've broken these out into two separate calls now.

+        base::TimeDelta::FromSecondsD([NSDate timeIntervalSinceReferenceDate] -
+                                      startTime));
+  };
+
   [self setLastTapPoint:sender];
   DCHECK(self.visible || self.dismissingModal);
   Tab* currentTab = [_model currentTab];
@@ -1625,6 +1645,11 @@ class BrowserBookmarkModelBridge : public bookmarks::BookmarkModelObserver {
             [self tabSelected:currentTab];
           }
           startVoiceSearchIfNecessaryBlock();
+
+          if (self.tabWasAddedCompletionBlock) {
+            self.tabWasAddedCompletionBlock();
+            self.tabWasAddedCompletionBlock = nil;
+          }
         });
   } else {
     // -updateSnapshotWithOverlay will force a screen redraw, so take the