fix build when ffmpeg, libvpx and libwebm are disabled

Tested with all combinaisons of
GYP_DEFINES="media_use_ffmpeg=i media_use_libvpx=j media_use_libwebm=k"
with i,j,k in {0, 1}^3

Same with GN:
media_use_ffmpeg = i
media_use_libvpx = j
media_use_libwebm = k
with i,j,k in {false, true}^3

If disabled it shows a black rectangle on video tag as expected.
If only ffmpeg is disabled, it succeeds to play webm video encoded with vpx.

BUG=
R=reillyg@chromium.org, jochen@chromium.org, xhwang@chromium.org, raymes@chromium.org
TEST=GYP_DEFINES="media_use_ffmpeg=0 media_use_libvpx=0 media_use_libwebm=0"

Review URL: https://codereview.chromium.org/1415793003

Cr-Commit-Position: refs/heads/master@{#359855}

• AUTHORS[diff]
• build/gn_migration.gypi[diff]
• chrome/utility/extensions/extensions_handler.cc[diff]
• chrome/utility/media_galleries/media_metadata_parser.cc[diff]
• content/renderer/media/audio_decoder.cc[diff]
• content/renderer/media/media_recorder_handler.cc[diff]
• content/renderer/media/media_recorder_handler.h[diff]
• content/renderer/pepper/video_decoder_shim.cc[diff]
• media/BUILD.gn[diff]
• media/blink/webmediaplayer_impl.cc[diff]
• media/media.gyp[diff]