Add MediaChunkIterator for HLS media playlists.

This allows to iterate through the available segments of the playlist.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204100116

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/playlist/HlsMediaPlaylistSegmentIterator.java

+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.source.hls.playlist;
+
+import android.net.Uri;
+import com.google.android.exoplayer2.source.chunk.BaseMediaChunkIterator;
+import com.google.android.exoplayer2.source.chunk.MediaChunkIterator;
+import com.google.android.exoplayer2.upstream.DataSpec;
+import com.google.android.exoplayer2.util.UriUtil;
+
+/** {@link MediaChunkIterator} wrapping a {@link HlsMediaPlaylist}. */
+public final class HlsMediaPlaylistSegmentIterator extends BaseMediaChunkIterator {
+
+  private final HlsMediaPlaylist playlist;
+  private final long startOfPlaylistInPeriodUs;
+
+  /**
+   * Creates iterator.
+   *
+   * @param playlist The {@link HlsMediaPlaylist} to wrap.
+   * @param startOfPlaylistInPeriodUs The start time of the playlist in the period, in microseconds.
+   * @param chunkIndex The chunk index in the playlist at which the iterator will start.
+   */
+  public HlsMediaPlaylistSegmentIterator(
+      HlsMediaPlaylist playlist, long startOfPlaylistInPeriodUs, int chunkIndex) {
+    super(/* fromIndex= */ chunkIndex, /* toIndex= */ playlist.segments.size() - 1);
+    this.playlist = playlist;
+    this.startOfPlaylistInPeriodUs = startOfPlaylistInPeriodUs;
+  }
+
+  @Override
+  public DataSpec getDataSpec() {
+    checkInBounds();
+    HlsMediaPlaylist.Segment segment = playlist.segments.get((int) getCurrentIndex());
+    Uri chunkUri = UriUtil.resolveToUri(playlist.baseUri, segment.url);
+    return new DataSpec(
+        chunkUri, segment.byterangeOffset, segment.byterangeLength, /* key= */ null);
+  }
+
+  @Override
+  public long getChunkStartTimeUs() {
+    checkInBounds();
+    HlsMediaPlaylist.Segment segment = playlist.segments.get((int) getCurrentIndex());
+    return startOfPlaylistInPeriodUs + segment.relativeStartTimeUs;
+  }
+
+  @Override
+  public long getChunkEndTimeUs() {
+    checkInBounds();
+    HlsMediaPlaylist.Segment segment = playlist.segments.get((int) getCurrentIndex());
+    long segmentStartTimeInPeriodUs = startOfPlaylistInPeriodUs + segment.relativeStartTimeUs;
+    return segmentStartTimeInPeriodUs + segment.durationUs;
+  }
+}