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

base/android/java/src/org/chromium/base/DiscardableReferencePool.java

Index: base/android/java/src/org/chromium/base/DiscardableReferencePool.java
diff --git a/base/android/java/src/org/chromium/base/DiscardableReferencePool.java b/base/android/java/src/org/chromium/base/DiscardableReferencePool.java
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+// Copyright 2017 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.base;
+
+import android.support.annotation.Nullable;
+
+import java.util.Collections;
+import java.util.Set;
+import java.util.WeakHashMap;
+
+/**
+ * A DiscardableReferencePool allows handing out typed references to objects ("payloads") that can
+ * be dropped in one batch ("drained"), e.g. under memory pressure. In contrast to {@link
+ * java.lang.ref.WeakReference}s, which drop their referents when they get garbage collected, a
+ * reference pool gives more precise control over when exactly it is drained.
+ *
+ * <p>Internally it uses a {@link WeakHashMap} with the reference itself as a key to allow the
+ * payloads to be garbage collected regularly when the last reference goes away before the pool is
+ * drained.
+ *
+ * <p>This class and its references are not thread-safe and should not be used simultaneously by
+ * multiple threads.
+ */
+public class DiscardableReferencePool {
+    /**
+     * The underlying data storage. The wildcard type parameter allows using a single pool for
+     * references of any type.
+     */
+    private final Set<DiscardableReference<?>> mPool;
+
+    public DiscardableReferencePool() {
+        WeakHashMap<DiscardableReference<?>, Boolean> map = new WeakHashMap<>();
+        mPool = Collections.newSetFromMap(map);
+    }
+
+    /**
+     * A reference to an object in the pool. Will be nulled out when the pool is drained.
+     * @param <T> The type of the object.
+     */
+    public static class DiscardableReference<T> {
+        @Nullable
+        private T mPayload;
+
+        private DiscardableReference(T payload) {
+            assert payload != null;
+            mPayload = payload;
+        }
+
+        /**
+         * @return The referent, or null if the pool has been drained.
+         */
+        @Nullable
+        public T get() {
+            return mPayload;
+        }
+
+        /**
+         * Clear the referent.
+         */
+        private void discard() {
+            assert mPayload != null;
+            mPayload = null;
+        }
+    }
+
+    /**
+     * @param <T> The type of the object.
+     * @param payload The payload to add to the pool.
+     * @return A new reference to the {@code payload}.
+     */
+    public <T> DiscardableReference<T> put(T payload) {
+        assert payload != null;
+        DiscardableReference<T> reference = new DiscardableReference<>(payload);
+        mPool.add(reference);
+        return reference;
+    }
+
+    /**
+     * Drains the pool, removing all references to objects in the pool and therefore allowing them
+     * to be garbage collected.
+     */
+    public void drain() {
+        for (DiscardableReference<?> ref : mPool) {
+            ref.discard();
+        }
+        mPool.clear();
+    }
+}