[Suggestions UI] Drop Bitmap references from articles under memory pressure.

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

 // Copyright 2016 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.
 package org.chromium.chrome.browser.ntp.snippets;
 
 import android.graphics.Bitmap;
 import android.support.annotation.Nullable;
 
+import org.chromium.base.DiscardableReferencePool.DiscardableReference;
 import org.chromium.chrome.browser.suggestions.OfflinableSuggestion;
 
 import java.io.File;
 
 /**
  * Represents the data for an article card on the NTP.
  */
 public class SnippetArticle implements OfflinableSuggestion {
     /** The category of this article. */
     public final int mCategory;
@@ skipping 25 matching lines @@
      */
     public final long mFetchTimestampMilliseconds;
 
     /** The rank of this article within its section. */
     private int mPerSectionRank = -1;
 
     /** The global rank of this article in the complete list. */
     private int mGlobalRank = -1;
 
     /** Bitmap of the thumbnail, fetched lazily, when the RecyclerView wants to show the snippet. */
-    private Bitmap mThumbnailBitmap;
+    private DiscardableReference<Bitmap> mThumbnailBitmap;
 
     /** Stores whether impression of this article has been tracked already. */
     private boolean mImpressionTracked;
 
     /** Whether the linked article represents an asset download. */
     private boolean mIsAssetDownload;
 
     /** The GUID of the asset download (only for asset download articles). */
     private String mAssetDownloadGuid;
 
@@ skipping 36 matching lines @@
     @Override
     public int hashCode() {
         return mCategory ^ mIdWithinCategory.hashCode();
     }
 
     /**
      * Returns this article's thumbnail as a {@link Bitmap}. Can return {@code null} as it is
      * initially unset.
      */
     public Bitmap getThumbnailBitmap() {
-        return mThumbnailBitmap;
+        return mThumbnailBitmap == null ? null : mThumbnailBitmap.get();
     }
 
     /** Sets the thumbnail bitmap for this article. */
-    public void setThumbnailBitmap(Bitmap bitmap) {
+    public void setThumbnailBitmap(DiscardableReference<Bitmap> bitmap) {
         mThumbnailBitmap = bitmap;
     }
 
     /** Returns whether to track an impression for this article. */
     public boolean trackImpression() {
         // Track UMA only upon the first impression per life-time of this object.
         if (mImpressionTracked) return false;
         mImpressionTracked = true;
         return true;
     }
@@ skipping 121 matching lines @@
     }
 
     public int getGlobalRank() {
         return mGlobalRank;
     }
 
     public int getPerSectionRank() {
         return mPerSectionRank;
     }
 }