media: Enable video decode acceleration through VAAPI

This CL enables H.264 codec video decode, leveraged by VAAPI. This work was
originally made for Crosswalk runtime by Qing Zhang <qing.zhang@intel.com> and
now been ported here with many additional changes.

The commands used to download, build and test libva:

  git clone git://anongit.freedesktop.org/libva
  cd libva/
  git checkout libva-1.2.1
  ./autogen.sh --prefix=$WLD  # you need to set your installation prefix here
  make -j4 && make install

  cd ../
  git clone git://anongit.freedesktop.org/vaapi/intel-driver
  cd intel-driver/
  git checkout 1.2.1
  ./autogen.sh --prefix=$WLD  # you need to set your installation prefix here
  make -j4 && make install

then on Chromium side, targets have to be build with something like:

  export GYP_DEFINES='chromeos=0 use_ozone=1 proprietary_codecs=1 ffmpeg_branding=Chrome'
  ninja -j16 -C out/Release content

and to run:
  ~/git/chromium/src/out/Release$ ./content_shell --no-sandbox --user-data-dir=/tmp/chrome --ignore-gpu-blacklist ../../third_party/WebKit/PerformanceTests/resources/bear-1280x720.mp4  --vmodule=*/ozone/ui*=3 --v=0

additionally, you can start content_shell with --disable-accelerated-video-decode
to compare the results.