Document usage of MATCH_LONGEST_US.

Handling of this constant is already in place everywhere, this
just documents that its use is allowed.

library/src/main/java/com/google/android/exoplayer/C.java

@@ -30,6 +30,12 @@ public final class C {
   public static final long UNKNOWN_TIME_US = -1L;
 
   /**
+   * Represents a microsecond duration whose exact value is unknown, but which should match the
+   * longest of some other known durations.
+   */
+  public static final long MATCH_LONGEST_US = -2L;
+
+  /**
    * The number of microseconds in one second.
    */
   public static final long MICROS_PER_SECOND = 1000000L;

library/src/main/java/com/google/android/exoplayer/TrackInfo.java

@@ -26,14 +26,17 @@ public final class TrackInfo {
   public final String mimeType;
 
   /**
-   * The duration in microseconds, or {@link C#UNKNOWN_TIME_US} if the duration is unknown.
+   * The duration in microseconds, or {@link C#UNKNOWN_TIME_US} if the duration is unknown, or
+   * {@link C#MATCH_LONGEST_US} if the duration should match the duration of the longest track whose
+   * duration is known.
    */
   public final long durationUs;
 
   /**
    * @param mimeType The mime type.
    * @param durationUs The duration in microseconds, or {@link C#UNKNOWN_TIME_US} if the duration
-   *     is unknown.
+   *     is unknown, or {@link C#MATCH_LONGEST_US} if the duration should match the duration of the
+   *     longest track whose duration is known.
    */
   public TrackInfo(String mimeType, long durationUs) {
     this.mimeType = mimeType;

library/src/main/java/com/google/android/exoplayer/TrackRenderer.java

@@ -70,9 +70,9 @@ public abstract class TrackRenderer implements ExoPlayerComponent {
   public static final long UNKNOWN_TIME_US = C.UNKNOWN_TIME_US; // -1
   /**
    * Represents a time or duration that should match the duration of the longest track whose
-   * duration is known.
+   * duration is known. Equal to {@link C#MATCH_LONGEST_US}.
    */
-  public static final long MATCH_LONGEST_US = -2;
+  public static final long MATCH_LONGEST_US = C.MATCH_LONGEST_US; // -2
   /**
    * Represents the time of the end of the track.
    */

library/src/main/java/com/google/android/exoplayer/chunk/SingleSampleChunkSource.java

@@ -44,7 +44,8 @@ public class SingleSampleChunkSource implements ChunkSource {
    * @param dataSpec Defines the location of the sample.
    * @param format The format of the sample.
    * @param durationUs The duration of the sample in microseconds, or {@link C#UNKNOWN_TIME_US} if
-   *     the duration is unknown.
+   *     the duration is unknown, or {@link C#MATCH_LONGEST_US} if the duration should match the
+   *     duration of the longest track whose duration is known.
    * @param mediaFormat The sample media format. May be null.
    */
   public SingleSampleChunkSource(DataSource dataSource, DataSpec dataSpec, Format format,