This is needed for at least the AudioFrame class.
Unfortunately, this does add a bit of ugliness because
module_common_types.h has video bits that are hidden behind our own
define, which now becomes part of pkg-config CFLAGS.
This could be made less ugly, potentially, but I'm not sure how right
now.
Corresponds to upstream commit 524e9b043e7e86fd72353b987c9d5f6a1ebf83e1
Update notes:
* Moved src/ to webrtc/ to easily diff against the third_party/webrtc
in the chromium tree
* ARM/NEON/MIPS support is not yet hooked up
* Tests have not been copied
