Roll src/third_party/ffmpeg/ 90cf969d6..68f193209 (2 commits)

https://chromium.googlesource.com/chromium/third_party/ffmpeg.git/+log/90cf969d61d6..68f1932090d5

$ git log 90cf969d6..68f193209 --date=short --no-merges --format='%ad %ae %s'
2019-05-09 flim Revert "Revert "libopus: Add channel mapping 2 support in libopusdec""
2019-05-06 msalama Default stack frames are needed on Android for the Orderfile.

Created with:
  roll-dep src/third_party/ffmpeg

Bug: chromium:958645
Change-Id: I1e59b53c569de513cd2b097021e69937aba41924
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607081Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658824}

DEPS

@@ -862,7 +862,7 @@ deps = {
   },
 
   'src/third_party/ffmpeg':
-    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '90cf969d61d6a0644c84a37e24156f1ddb6a208d',
+    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '68f1932090d5e35d958434d0588a33986cd3d65d',
 
   'src/third_party/flac':
     Var('chromium_git') + '/chromium/deps/flac.git' + '@' + 'af862024c8c8fa0ae07ced05e89013d881b00596',