This patch addes trampoline headers for libvpx.
To follow the libwebrtc merge procedure, the vpx headers are silently
replaced with "trampoline" headers, which do nothing but include real
VPX headers. This makes the libwebrtc-merge process easier without
worrying headers' paths.
On the other hand, the rtc_build_libvpx is set to true in
webrtc.gni, so moz.build file for third_party/libvpx can be generated by
the gn_processor in the next patch.