Allow DashMediaSource to take an optional DashManifest

Also allow custom DashManifestParser injection, to
support parsing of custom elements and attributes that
service providers may wish to include in their manifests
(e.g. #2058).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139813108

library/src/main/java/com/google/android/exoplayer2/upstream/LoaderErrorThrower.java

@@ -43,4 +43,21 @@ public interface LoaderErrorThrower {
    */
   void maybeThrowError(int minRetryCount) throws IOException;
 
+  /**
+   * A {@link LoaderErrorThrower} that never throws.
+   */
+  final class Dummy implements LoaderErrorThrower {
+
+    @Override
+    public void maybeThrowError() throws IOException {
+      // Do nothing.
+    }
+
+    @Override
+    public void maybeThrowError(int minRetryCount) throws IOException {
+      // Do nothing.
+    }
+
+  }
+
 }