[NTP Snippets] Add UMA for snippet dismissal

BUG=603576

Review URL: https://codereview.chromium.org/1882953004

Cr-Commit-Position: refs/heads/master@{#387576}

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

@@ -540,6 +540,7 @@ public void onLoadingComplete(MostVisitedItem[] items) {
         @Override
         public void onSnippetDismissed(SnippetArticle dismissedSnippet) {
             if (mIsDestroyed) return;
+            NewTabPageUma.recordSnippetAction(NewTabPageUma.SNIPPETS_ACTION_DISMISSED);
             mSnippetsBridge.discardSnippet(dismissedSnippet);
         }
     };

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

@@ -61,7 +61,8 @@ public class NewTabPageUma {
     private static final int NUM_NTP_IMPRESSION = 2;
 
     /** Possible interactions with the snippets. */
-    @IntDef({SNIPPETS_ACTION_SHOWN, SNIPPETS_ACTION_SCROLLED, SNIPPETS_ACTION_CLICKED})
+    @IntDef({SNIPPETS_ACTION_SHOWN, SNIPPETS_ACTION_SCROLLED, SNIPPETS_ACTION_CLICKED,
+            SNIPPETS_ACTION_DISMISSED})
     @Retention(RetentionPolicy.SOURCE)
     public @interface SnippetsAction {}
     /** Snippets are enabled and are being shown to the user. */
@@ -70,8 +71,10 @@ public class NewTabPageUma {
     public static final int SNIPPETS_ACTION_SCROLLED = 1;
     /** A snippet has been clicked. */
     public static final int SNIPPETS_ACTION_CLICKED = 2;
+    /** A snippet has been swiped away. */
+    public static final int SNIPPETS_ACTION_DISMISSED = 3;
     /** The number of possible actions. */
-    private static final int NUM_SNIPPETS_ACTIONS = 3;
+    private static final int NUM_SNIPPETS_ACTIONS = 4;
 
     /**
      * Records an action taken by the user on the NTP.

tools/metrics/histograms/histograms.xml

@@ -83765,6 +83765,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="0" label="Snippets were shown to the user"/>
   <int value="1" label="User scrolled through the snippets"/>
   <int value="2" label="User clicked on a snippet into the host website"/>
+  <int value="3" label="User swiped a snippet away"/>
 </enum>
 
 <enum name="SocketStreamConnectionType" type="int">