Fix that NewTabPage.Snippets.CardShown was recorded too early/often.

chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
index c4ef6231b4b10765dedc24b43069edae62be04c7..9e8816656de283cdf913310e6bc11a4557cbc1e0 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/snippets/SnippetArticle.java
@@ -5,6 +5,7 @@ package org.chromium.chrome.browser.ntp.snippets;
 
 import android.graphics.Bitmap;
 
+import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.chrome.browser.ntp.cards.NewTabPageListItem;
 
 /**
@@ -25,6 +26,9 @@ public class SnippetArticle implements NewTabPageListItem {
     /** Bitmap of the thumbnail, fetched lazily, when the RecyclerView wants to show the snippet. */
     private Bitmap mThumbnailBitmap;
 
+    /** Stores whether impression of this article has been tracked already. */
+    private boolean mImpressionTracked;
+
     /**
      * Creates a SnippetArticle object that will hold the data
      * @param title the title of the article
@@ -79,4 +83,18 @@ public class SnippetArticle implements NewTabPageListItem {
     public void setThumbnailBitmap(Bitmap bitmap) {
         mThumbnailBitmap = bitmap;
     }
+
+    /** Tracks impression of this NTP snippet. */
+    public void trackImpression() {
+        // Track UMA only upon the first impression per life-time of this object.
+        if (mImpressionTracked) return;
+
+        RecordHistogram.recordSparseSlowlyHistogram("NewTabPage.Snippets.CardShown", mPosition);
+        mImpressionTracked = true;
+    }
+
+    /** Returns whether impression of this SnippetArticle has already been tracked. */
+    public boolean impressionTracked() {
+        return mImpressionTracked;
+    }
 }