Remove usage of Tab's |url| property from NewTabPageUMA utilities.

ios/chrome/browser/metrics/new_tab_page_uma.mm

Index: ios/chrome/browser/metrics/new_tab_page_uma.mm
diff --git a/ios/chrome/browser/metrics/new_tab_page_uma.mm b/ios/chrome/browser/metrics/new_tab_page_uma.mm
index 8405afa190e9ee123388a1c17f02fd58ae0b7163..c07308925cd50309b2857f6c87eb7b7c66b512bf 100644
--- a/ios/chrome/browser/metrics/new_tab_page_uma.mm
+++ b/ios/chrome/browser/metrics/new_tab_page_uma.mm
@@ -15,10 +15,11 @@
 
 namespace new_tab_page_uma {
 
-bool IsCurrentlyOnNTP(ios::ChromeBrowserState* browserState) {
-  TabModel* tabModel = GetLastActiveTabModelForChromeBrowserState(browserState);
-  return tabModel.currentTab &&
-         tabModel.currentTab.url == GURL(kChromeUINewTabURL);
+bool IsCurrentlyOnNTP(ios::ChromeBrowserState* browser_state) {
+  TabModel* tab_model =
+      GetLastActiveTabModelForChromeBrowserState(browser_state);
+  return tab_model.currentTab &&
+         tab_model.currentTab.visibleURL == GURL(kChromeUINewTabURL);

[kkhorimoto, 2017/04/14 20:34:33]

The old behavior would return a non-NTP URL if there was any navigation
occurring at all.  This updates the implementation slightly to return the
non-NTP URL only if there's a user-initiated load.

[Eugene But (OOO till 7-30), 2017/04/14 21:43:02]

Should this be lastCommittedURL? I think that old code was not correct.

[kkhorimoto, 2017/05/31 22:58:33]

For user-initiated loads, the web view will be shown immediately and the NTP
will be removed as soon as the pending item is created.  Thus, if the visible
item's URL is not the NTP, then the NTP is not currently being displayed.  For
renderer-initiated loads, the web view will be shown only after the load is
committed.  Using the visible URL makes sense to me because it's tightly coupled
to whether the NTP or the webview is currently displayed.

 }
 
 void RecordAction(ios::ChromeBrowserState* browserState, ActionType type) {