Roll src/third_party/ffmpeg/ 4e878f7f6..75976ae02 (1 commit).

https://chromium.googlesource.com/chromium/third_party/ffmpeg.git/+log/4e878f7f64d7..75976ae026fd

$ git log 4e878f7f6..75976ae02 --date=short --no-merges --format='%ad %ae %s'
2016-08-03 michael avformat/oggdec: Fix integer overflow with invalid pts

BUG=614034

Review-Url: https://codereview.chromium.org/2224123002
Cr-Commit-Position: refs/heads/master@{#410773}

DEPS

@@ -187,7 +187,7 @@ deps = {
    Var('chromium_git') + '/webm/libvpx.git' + '@' +  '82070ae9393b1e79559d81fcf1aa89c2e4aa58ee',
 
   'src/third_party/ffmpeg':
-   Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '4e878f7f64d7a9f06ba076d0a2611d7f8b33d793',
+   Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '75976ae026fdbedb14f006eec6cd9119c543aa7f',
 
   'src/third_party/usrsctp/usrsctplib':
     Var('chromium_git') + '/external/github.com/sctplab/usrsctp' + '@' + '9a3e5465e9d96d8a7f78f1e996412d6235d7a359',