Enable buffering for CacheDataSink by default

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146221487

library/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSink.java

@@ -32,6 +32,9 @@
  */
 public final class CacheDataSink implements DataSink {
 
+  /** Default buffer size. */
+  public static final int DEFAULT_BUFFER_SIZE = 20480;
+
   private final Cache cache;
   private final long maxCacheFileSize;
   private final int bufferSize;
@@ -56,13 +59,15 @@ public CacheDataSinkException(IOException cause) {
   }
 
   /**
+   * Constructs a CacheDataSink using the {@link #DEFAULT_BUFFER_SIZE}.
+   *
    * @param cache The cache into which data should be written.
    * @param maxCacheFileSize The maximum size of a cache file, in bytes. If the sink is opened for
    *    a {@link DataSpec} whose size exceeds this value, then the data will be fragmented into
    *    multiple cache files.
    */
   public CacheDataSink(Cache cache, long maxCacheFileSize) {
-    this(cache, maxCacheFileSize, 0);
+    this(cache, maxCacheFileSize, DEFAULT_BUFFER_SIZE);
   }
 
   /**

library/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSinkFactory.java

@@ -24,18 +24,27 @@ public final class CacheDataSinkFactory implements DataSink.Factory {
 
   private final Cache cache;
   private final long maxCacheFileSize;
+  private final int bufferSize;
 
   /**
    * @see CacheDataSink#CacheDataSink(Cache, long)
    */
   public CacheDataSinkFactory(Cache cache, long maxCacheFileSize) {
+    this(cache, maxCacheFileSize, CacheDataSink.DEFAULT_BUFFER_SIZE);
+  }
+
+  /**
+   * @see CacheDataSink#CacheDataSink(Cache, long, int)
+   */
+  public CacheDataSinkFactory(Cache cache, long maxCacheFileSize, int bufferSize) {
     this.cache = cache;
     this.maxCacheFileSize = maxCacheFileSize;
+    this.bufferSize = bufferSize;
   }
 
   @Override
   public DataSink createDataSink() {
-    return new CacheDataSink(cache, maxCacheFileSize);
+    return new CacheDataSink(cache, maxCacheFileSize, bufferSize);
   }
 
 }