Revert of Roll ffmpeg 3540f6b0:be6eebd8 (patchset #1 id:1 of https://codereview.chromium.org/1447343002/ )
Revert of Roll ffmpeg 3540f6b0:be6eebd8 (patchset #1 id:1 of https://codereview.chromium.org/1447343002/ )

Reason for revert:
This broke the non-ToT Win Clang bots, e.g. http://build.chromium.org/p/chromium.fyi/builders/CrWinClang64/builds/2443/steps/compile/logs/stdio

Original issue's description:
> Roll ffmpeg 3540f6b0:be6eebd8
>
> Picks up be6eebd8: "Don't use the _tzcnt intrinsics"
>
> BUG=556735
>
> Committed: https://crrev.com/a2e8a4030fc3da64a9d3c261fc5402213dcc443d
> Cr-Commit-Position: refs/heads/master@{#359987}

TBR=thakis@chromium.org,dalecurtis@google.com,ddorwin@google.com,ddorwin@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=556735

Review URL: https://codereview.chromium.org/1455623002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360080}

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