Use preprocessor to strip H264 implementation.

This CL makes it more flexible and easier to include/exclude H264 code
when using other build systems because it delegates the decision to
remove the code to the preprocessor instead of GN.

This CL should be a noop, and for WebRTC/Chromium the GN param
`rtc_use_h264` will still be the only thing to change in order to
include/exclude H264.

Moving code that requires ffmpeg or h264 out of the #ifdef/#endif
part should break the build since dependencies are only added if
`rtc_use_h264=true`.

Bug: webrtc:9213
Change-Id: Ibc04edc2f6b9e51489ffe638d5be4b32959cdca0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/137430
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28055}

modules/video_coding/codecs/h264/h264_decoder_impl.cc

@@ -9,6 +9,11 @@
  *
  */
 
+// Everything declared/defined in this header is only required when WebRTC is
+// build with H264 support, please do not move anything out of the
+// #ifdef unless needed and tested.
+#ifdef WEBRTC_USE_H264
+
 #include "modules/video_coding/codecs/h264/h264_decoder_impl.h"
 
 #include <algorithm>
@@ -380,3 +385,5 @@ void H264DecoderImpl::ReportError() {
 }
 
 }  // namespace webrtc
+
+#endif  // WEBRTC_USE_H264