webrtc doesnt include the headers that the functions are prototyped in.
This CL makes the convert.h include those headers to allow webrtc to
update to the head libyuv.

TBR=harryjin@google.com
BUG=libyuv:620,webrtc:6091,webrtc:6094
TESTED=local build and try bots

Review URL: https://codereview.chromium.org/2141683002 .