Switch various media #defines to BUILDFLAGS.

Converts the following to buildflags:
- MEDIA_DISABLE_FFMPEG becomes ENABLE_FFMPEG, but keeps media_use_ffmpeg
as the GN variable due to external users.
- MEDIA_DISABLE_LIBVPX becomes ENABLE_LIBVPX, but keeps media_use_libvpx
as the GN variable due to external users.
- DISABLE_FFMPEG_VIDEO_DECODERS becomes ENABLE_FFMPEG_VIDEO_DECODERS and
the GN variable changes to enable_ffmpeg_video_decoders due to no
external users AFAIK.

Some incorrect usages of just MEDIA_DISABLE_FFMPEG to gate H264 support
and some less-efficient usages of !MEDIA_DISABLE_FFMPEG + !ANDROID are
replaced with the correct ENABLE_FFMPEG_VIDEO_DECODERS instead.

BUG=596252
TEST=compiles normally

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Iab4548feabab60d7fbed4144457f0be9f2e98f47
Reviewed-on: https://chromium-review.googlesource.com/865520
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Thomas Guilbert <[email protected]>
Commit-Queue: Dale Curtis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#529559}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index cca8193..6e2f1098 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2605,12 +2605,13 @@
      flag_descriptions::kUserActivationV2Description, kOsAll,
      FEATURE_VALUE_TYPE(features::kUserActivationV2)},
 #if BUILDFLAG(ENABLE_WEBRTC) && BUILDFLAG(RTC_USE_H264) && \
-    !defined(MEDIA_DISABLE_FFMPEG)
+    BUILDFLAG(ENABLE_FFMPEG_VIDEO_DECODERS)
     {"enable-webrtc-h264-with-openh264-ffmpeg",
      flag_descriptions::kWebrtcH264WithOpenh264FfmpegName,
      flag_descriptions::kWebrtcH264WithOpenh264FfmpegDescription, kOsDesktop,
      FEATURE_VALUE_TYPE(content::kWebRtcH264WithOpenH264FFmpeg)},
-#endif  // ENABLE_WEBRTC && BUILDFLAG(RTC_USE_H264) && !MEDIA_DISABLE_FFMPEG
+#endif  // ENABLE_WEBRTC && BUILDFLAG(RTC_USE_H264) &&
+        // BUILDFLAG(ENABLE_FFMPEG_VIDEO_DECODERS)
 #if defined(OS_ANDROID)
     {"offlining-recent-pages", flag_descriptions::kOffliningRecentPagesName,
      flag_descriptions::kOffliningRecentPagesDescription, kOsAndroid,