Fix release shader resources in the right OpenGL context

Bug: webrtc:8705
Change-Id: I772d86b33fdc7903d874e6ba37e63dd53be6f08e
Reviewed-on: https://webrtc-review.googlesource.com/37082
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21519}
diff --git a/sdk/objc/Framework/Classes/UI/RTCEAGLVideoView.m b/sdk/objc/Framework/Classes/UI/RTCEAGLVideoView.m
index 53f5270..8f379c8 100644
--- a/sdk/objc/Framework/Classes/UI/RTCEAGLVideoView.m
+++ b/sdk/objc/Framework/Classes/UI/RTCEAGLVideoView.m
@@ -197,6 +197,8 @@
     [self teardownGL];
   }
   [_timer invalidate];
+  [self ensureGLContext];
+  _shader = nil;
   if (_glContext && [EAGLContext currentContext] == _glContext) {
     [EAGLContext setCurrentContext:nil];
   }