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}

DEPS

@@ -117,7 +117,7 @@ deps = {
       'condition': 'checkout_android',
   },
   'src/third_party/ffmpeg':
-    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '90210b5e10d3917567a3025e4853704bfefd8384',
+    Var('chromium_git') + '/chromium/third_party/ffmpeg.git' + '@' + '42bb040dde0a5e1bdb4b6d91f54297e0cc7f0977',
   'src/third_party/findbugs': {
     'url': Var('chromium_git') + '/chromium/deps/findbugs.git' + '@' + '4275d9ac8610db6b1bc9a5e887f97e41b33fac67',
     'condition': 'checkout_android',