Cleanup outdated comment regarding applyConstraints().

Looking at the referenced bug (crbug/338503) it looks like this comment
is outdated since the spec was already implemented. This CL removes the
comment.

BUG=338503

Change-Id: Id986c398ea59ea1d50c0835567a160b900769cd0
Reviewed-on: https://chromium-review.googlesource.com/1188566Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586242}

content/renderer/media/stream/user_media_client_impl.cc

@@ -181,7 +181,7 @@ void UserMediaClientImpl::RequestUserMedia(
 void UserMediaClientImpl::ApplyConstraints(
     const blink::WebApplyConstraintsRequest& web_request) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  // TODO(guidou): Implement applyConstraints(). http://crbug.com/338503
+
   pending_request_infos_.push_back(Request(web_request));
   if (!is_processing_request_)
     MaybeProcessNextRequestInfo();