Remove outdated comment in blink::HTMLMediaElement

http://crrev/0f21e26 did a mechanical replacement of
|TraceWrapperMember| to |Member|, and this comment is no longer relevant
(and also doesn't make any sense!)

Change-Id: I77b888c5ec1092be392742e0651dafc43bcbe4ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067280Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Will Cassella <cassew@google.com>
Commit-Queue: Will Cassella <cassew@google.com>
Cr-Commit-Position: refs/heads/master@{#744337}

third_party/blink/renderer/core/html/media/html_media_element.h

@@ -626,13 +626,6 @@ class CORE_EXPORT HTMLMediaElement
 
   DisplayMode display_mode_;
 
-  // If any portion of an attached HTMLMediaElement (HTMLME) and the MediaSource
-  // Extensions (MSE) API is alive (having pending activity or traceable from a
-  // GC root), the whole group is not GC'ed. Here, using Member, because
-  // |media_source_|'s wrapper needs to remain alive at least to successfully
-  // dispatch any events enqueued by behavior of the HTMLME+MSE API. It makes
-  // |media_source_|'s wrapper remain alive as long as this HTMLMediaElement's
-  // wrapper is alive.
   Member<MediaSource> media_source_;
 
   // Stores "official playback position", updated periodically from "current