Send index value from content widget.

This value was assumed received on the histogram side but not sent,
resulting in an empty histogram.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I58b4e97eca73f987db6380485d34ffe594329e1e
Reviewed-on: https://chromium-review.googlesource.com/758855Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Elodie Banel <lod@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515518}

ios/chrome/content_widget_extension/content_widget_view_controller.mm

@@ -198,6 +198,8 @@ const CGFloat widgetCompactHeightIOS9 = 110;
       app_group::kChromeAppGroupCommandCommandPreference);
   NSString* URLPrefKey =
       base::SysUTF8ToNSString(app_group::kChromeAppGroupCommandURLPreference);
+  NSString* indexKey =
+      base::SysUTF8ToNSString(app_group::kChromeAppGroupCommandIndexPreference);
 
   NSDictionary* commandDict = @{
     timePrefKey : [NSDate date],
@@ -205,6 +207,7 @@ const CGFloat widgetCompactHeightIOS9 = 110;
     commandPrefKey :
         base::SysUTF8ToNSString(app_group::kChromeAppGroupOpenURLCommand),
     URLPrefKey : URL.absoluteString,
+    indexKey : [NSNumber numberWithInt:[self.sites objectForKey:URL].position]
   };
 
   [sharedDefaults setObject:commandDict forKey:defaultsKey];