Add new NTP layout with snippet cards and hide it behind a flag

chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java
index 60681f11fd75211808486b0a13d0461a3f307f39..28edcc41ac84bf1999d389649836d38a7314a463 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageUma.java
@@ -4,12 +4,17 @@
 
 package org.chromium.chrome.browser.ntp;
 
+import android.support.annotation.IntDef;
+
 import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.base.metrics.RecordUserAction;
 import org.chromium.chrome.browser.rappor.RapporServiceBridge;
 import org.chromium.chrome.browser.util.UrlUtilities;
 import org.chromium.ui.base.PageTransition;
 
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
 /**
  * Records UMA stats for which actions the user takes on the NTP in the
  * "NewTabPage.ActionAndroid" histogram.
@@ -46,6 +51,19 @@
     // The number of possible actions pertinent to Rappor
     private static final int RAPPOR_NUM_ACTIONS = 2;
 
+    /** Possible interactions with the snippets. */
+    @IntDef({SNIPPETS_ACTION_SHOWN, SNIPPETS_ACTION_SCROLLED, SNIPPETS_ACTION_CLICKED})
+    @Retention(RetentionPolicy.SOURCE)
+    public @interface SnippetsAction {}
+    /** Snippets are enabled and are being shown to the user. */
+    public static final int SNIPPETS_ACTION_SHOWN = 0;
+    /** The snippet list has been scrolled. */
+    public static final int SNIPPETS_ACTION_SCROLLED = 1;
+    /** A snippet has been clicked. */
+    public static final int SNIPPETS_ACTION_CLICKED = 2;
+    /** The number of possible actions. */
+    private static final int NUM_SNIPPETS_ACTIONS = 3;
+
     /**
      * Records an action taken by the user on the NTP.
      * @param action One of the ACTION_* values defined in this class.
@@ -108,4 +126,13 @@ public static void recordExplicitUserNavigation(String destinationUrl, int rappo
                 return;
         }
     }
+
+    /**
+     * Records important events related to snippets.
+     * @param action action key, one of {@link SnippetsAction}'s values.
+     */
+    public static void recordSnippetAction(@SnippetsAction int action) {

[newt (away), 2016/03/30 19:03:50]

Did you consider using user actions here instead of a histogram? A histogram has
the benefit that it's easy to see all the related events in a single view with
relative frequencies. A user action has the benefit that you can see a timeline
view, showing what actions users most often perform before or after, say,
clicking on a snippet. (SNIPPETS_SHOWN isn't really a user action though...)

[dgn, 2016/03/31 00:46:44]

There is the issue that SNIPPETS_SHOWN isn't a user action, but the description
for that metric doesn't make it fit that well as histogram either. I'll ask
around about the expectations for this histogram, as I didn't change how it's
recorded, just moved it from SnippetManager

+        RecordHistogram.recordEnumeratedHistogram(
+                "NewTabPage.Snippets.Interactions", action, NUM_SNIPPETS_ACTIONS);
+    }
 }