[MediaRecorder] Resume WebmMuxer only if not null

There are some cases in which the muxer is null, which lead to crashes.

Bug: 954494
Change-Id: I571d54d56c71d7a674054979a441e6645bc5ac3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578614
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#653191}(cherry picked from commit bb460bf)
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1581684
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/branch-heads/3770@{#43}
Cr-Branched-From: a9eee1c-refs/heads/master@{#652427}

content/renderer/media_recorder/media_recorder_handler.cc

@@ -343,7 +343,8 @@ void MediaRecorderHandler::Resume() {
     video_recorder->Resume();
   for (const auto& audio_recorder : audio_recorders_)
     audio_recorder->Resume();
-  webm_muxer_->Resume();
+  if (webm_muxer_)
+    webm_muxer_->Resume();
 }
 
 void MediaRecorderHandler::EncodingInfo(