Roll src/third_party/ffmpeg/ 68b4dfdbd..252792a1f (2 commits)

https://chromium.googlesource.com/chromium/third_party/ffmpeg.git/+log/68b4dfdbd607..252792a1ff65

$ git log 68b4dfdbd..252792a1f --date=short --no-merges --format='%ad %ae %s'
2019-12-09 tguilbert Fix null pointer addition error
2019-12-06 tguilbert Fix null pointer arithmetic

Created with:
  roll-dep src/third_party/ffmpeg

Bug: 1031252
Change-Id: I63615b0744a5fac96358dccc6ba16bf62c6dedf4
TBR: tmathmeyer
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1960892Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Ted Meyer <tmathmeyer@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723596}

DEPS

@@ -911,7 +911,7 @@ deps = {
   },
 
   'src/third_party/ffmpeg':
-    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '68b4dfdbd607fdd33b9b8e3515f039d97677dd1c',
+    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '252792a1ff65deadc4e627e59ce80b8811c5a5de',
 
   'src/third_party/flac':
     Var('chromium_git') + '/chromium/deps/flac.git' + '@' + 'af862024c8c8fa0ae07ced05e89013d881b00596',