Add metric to record users opening a new NTP.

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is a comprehensive list of Chrome user actions along with the owner
 and description for each user action.
 
@@ skipping 9943 matching lines @@
     Deprecated as of 01/2017. Replaced with
     MobileRecentTabManagerTabFromOtherDeviceOpened.
   </obsolete>
 </action>
 
 <action name="MobileNTPMostVisited">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
 </action>
 
+<action name="MobileNTPOpenedInNewTab">
+  <owner>fhorschig@chromium.org</owner>
+  <description>
+    The user manually created a New Tab Page (NTP) in a new tab that has not

[Michael van Ouwerkerk, 2017/03/14 16:07:36]

Have you confirmed that this indeed is not logged at startup when an NTP is
created? Because in that case the user did not exactly manually create one.

[fhorschig, 2017/03/14 17:30:45]

Adjusted description to match my intention.
If you like, I can exclude the "manually", too but by opening chrome,
the user triggers that NTP. I know, the line is blurry here but I want it 
to be logged (and confirmed that), because that instantiation is never 
happening due to a navigation which is what I really want to figure out
by measuring this.
If it (surprisingly) turns out to be unwanted, we can use the LoadType 
metric to correct for cold starts. For warm starts, the navigation rules
should apply as usual.

+    been shown before.
+  </description>
+</action>
+
 <action name="MobileNTPRecentlyClosed">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>
   <obsolete>
     Deprecated as of 01/2017. Replaced with
     MobileRecentTabManagerRecentTabOpened.
   </obsolete>
 </action>
 
 <action name="MobileNTPShown">
@@ skipping 6546 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>