Remove some superfluous "custom"s.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192100852

RELEASENOTES.md

@@ -27,8 +27,8 @@
   * Added callbacks to `MediaSourceEventListener` to get notified when media
     periods are created, released and being read from.
   * Support live stream clipping with `ClippingMediaSource`.
-  * Allow setting custom tags for all media sources in their factories. The tag
-    of the current window can be retrieved with `ExoPlayer.getCurrentTag`.
+  * Allow setting tags for all media sources in their factories. The tag of the
+    current window can be retrieved with `ExoPlayer.getCurrentTag`.
 * Audio:
   * Factor out `AudioTrack` position tracking from `DefaultAudioSink`.
   * Fix an issue where the playback position would pause just after playback

library/core/src/main/java/com/google/android/exoplayer2/Timeline.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2;
 
+import android.support.annotation.Nullable;
 import android.util.Pair;
 import com.google.android.exoplayer2.source.ads.AdPlaybackState;
 import com.google.android.exoplayer2.util.Assertions;
@@ -118,8 +119,8 @@ public abstract class Timeline {
    */
   public static final class Window {
 
-    /** A custom tag for the window. Not necessarily unique. */
-    public Object tag;
+    /** A tag for the window. Not necessarily unique. */
+    public @Nullable Object tag;
 
     /**
      * The start time of the presentation to which this window belongs in milliseconds since the
@@ -174,7 +175,7 @@ public abstract class Timeline {
 
     /** Sets the data held by this window. */
     public Window set(
-        Object tag,
+        @Nullable Object tag,
         long presentationStartTimeMs,
         long windowStartTimeMs,
         boolean isSeekable,

library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java

@@ -54,7 +54,7 @@ public final class SinglePeriodTimeline extends Timeline {
    * @param durationUs The duration of the period, in microseconds.
    * @param isSeekable Whether seeking is supported within the period.
    * @param isDynamic Whether the window may change when the timeline is updated.
-   * @param tag A custom tag used for {@link Timeline.Window#tag}.
+   * @param tag A tag used for {@link Timeline.Window#tag}.
    */
   public SinglePeriodTimeline(
       long durationUs, boolean isSeekable, boolean isDynamic, @Nullable Object tag) {
@@ -80,7 +80,7 @@ public final class SinglePeriodTimeline extends Timeline {
    *     which to begin playback, in microseconds.
    * @param isSeekable Whether seeking is supported within the window.
    * @param isDynamic Whether the window may change when the timeline is updated.
-   * @param tag A custom tag used for {@link Timeline.Window#tag}.
+   * @param tag A tag used for {@link Timeline.Window#tag}.
    */
   public SinglePeriodTimeline(
       long periodDurationUs,
@@ -117,8 +117,7 @@ public final class SinglePeriodTimeline extends Timeline {
    *     which to begin playback, in microseconds.
    * @param isSeekable Whether seeking is supported within the window.
    * @param isDynamic Whether the window may change when the timeline is updated.
-   * @param tag A custom tag used for {@link Timeline.Window#tag}. If null, an arbitrary default tag
-   *     is used.
+   * @param tag A tag used for {@link Timeline.Window#tag}.
    */
   public SinglePeriodTimeline(
       long presentationStartTimeMs,