Add ability to get video stream.

Current status:
===========
  Extension with idl interface (done)
  Example (done, need update time to time)
  Extension : Able to compile/pack, need verify whether can save to test.h264 in cacheDir()

Plan:
===========
  1. Use mOption.latency() to generate seperate .h264
  2. Import mp4parser and mutex the .h264 file, and save as .mp4 in cache (the hardest one)
  3. Send "newvideoready" event when .mp4 file is ready

Command to build:
===========
gyp --depth=. all.gyp && \
ninja -C out/Default/ && make_apk.py \
--package=org.xwalk.extensions.ardrone \
--extensions="./out/Default/gen/ardrone_video/" --arch=x86 \
--manifest="./examples/ardrone_video/manifest.json" && \
adb install -r Ardrone_0.0.0.1_x86.apk

How to remote debugging:
===========
1. Apply
https://github.com/halton/crosswalk/commit/89a0d71b0094fe0c8f6bd63c092ae384504f4998
in src/xwalk
2. Rebuild xwalk_builder target
3. Pack app with make_apk.py under out/Debug/out/Debug/xwalk_app_template
4. Start app on mobile
5. Start chrome://inspect on your host PC