Roll //third_party/ffmpeg and disable H264 when MSVC is used.

FFmpeg hasn't been rolled since [1] in order to avoid to break MSVC
trybots (//third_party/ffmpeg dropped MSVC support, in theory it is
possible to bring the support back but some work is needed every time
//third_party/ffmpeg gets updated).

Not rolling //third_party/ffmpeg is not enough to keep the Chromium
Roll working because -Wstring-plus-int becomes more chatty with clang 350768
and it has been suppressed in //third_party/ffmpeg/BUILD.gn [2].

Since WebRTC needs to update clang, //third_party/ffmpeg needs to be
updated. The only way to do it without fixing MSVC errors in
//third_party/ffmpeg is to enforce rtc_use_h264=False when MSVC is used.

PSA: https://groups.google.com/forum/#!topic/discuss-webrtc/cfkPPq5nvNE.

[1] - https://webrtc-review.googlesource.com/78402
[2] - https://chromium-review.googlesource.com/c/chromium/third_party/ffmpeg/+/1376376

Bug: webrtc:9213
Change-Id: I36bd7fb2db21012760e4ff7a791d81350e402ec0
Reviewed-on: https://webrtc-review.googlesource.com/c/116982
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26257}

modules/video_coding/BUILD.gn

@@ -333,6 +333,7 @@ rtc_static_library("webrtc_h264") {
   ]
 
   if (rtc_use_h264) {
+    assert(!(is_win && !is_clang), "See: bugs.webrtc.org/9213#c13.")
     defines += [ "WEBRTC_USE_H264" ]
     sources += [
       "codecs/h264/h264_color_space.cc",