Fix uninitialized value

Set default position.

BUG=987381

Change-Id: I12fa8599c6b5a9630a252412261fc0edd24aadf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717306
Auto-Submit: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680607}

services/media_session/public/cpp/media_position.h

@@ -67,7 +67,7 @@ struct COMPONENT_EXPORT(MEDIA_SESSION_BASE_CPP) MediaPosition {
                            TestNotEquals_DifferentPlaybackRate);
 
   // Playback rate of the media.
-  double playback_rate_;
+  double playback_rate_ = 0;
 
   // Duration of the media.
   base::TimeDelta duration_;