Remove GSM from the list of FFmpeg formats checked

Now that FFmpeg is no longer configured to check for GSM containers, there
is no need to look for it in the FFmpeg context.

BUG=711774
TEST=media_unittests still pass (although there was no specific test for GSM)

Change-Id: I3d34010097be177554c31ac083341fbfee0a7cc9
Reviewed-on: https://chromium-review.googlesource.com/530164Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478823}

media/filters/ffmpeg_glue.cc

@@ -189,9 +189,6 @@ bool FFmpegGlue::OpenContext() {
     container_ = container_names::CONTAINER_AMR;
   else if (strcmp(format_context_->iformat->name, "avi") == 0)
     container_ = container_names::CONTAINER_AVI;
-  // TODO(jrummell): Remove GSM detection. http://crbug.com/711774
-  else if (strcmp(format_context_->iformat->name, "gsm") == 0)
-    container_ = container_names::CONTAINER_GSM;
 
   DCHECK_NE(container_, container_names::CONTAINER_UNKNOWN);
   UMA_HISTOGRAM_SPARSE_SLOWLY("Media.DetectedContainer", container_);