Merge changes I826655a7,I5164df72,Iba9b198c,Ide9a6846,I4f51ce85,I0e6aa00f,Ic334da9a,I252f5f8a,I7865db2d,I13b434b1

* changes:
  test/: remove unnecessary extern "C"s
  top-level: add extern "C" to headers
  vpx_ports: add extern "C" to headers
  vpx: add extern "C" to headers
  vp9/encoder: add extern "C" to headers
  vp9/decoder: add extern "C" to headers
  vp9/common: add extern "C" to headers
  vp8/encoder: add extern "C" to headers
  vp8/decoder: add extern "C" to headers
  vp8/common: add extern "C" to headers