[webapi][xwalk-2561] Update codes based on Chromium implement

- mp4a.40.8/mp4v.20.8 is also not supported in chromium 38, need use mp4a.40.2/mp4a.40 instead.
- Verified on Crosswalk 9.38.204.0

Impacted tests(approved): new 0, update 2, delete 0
Unit test platform: [Tizen IVI][Android]
Unit test result summary: pass 2, fail 0, block 0
[wrt][XWALK-1668]Add new audiopolicy testcase

Impacted TCs num(approved): New audiopolicy 7, update multiuser 3, deleted 0
Implacted TCs num(designed): New 2, Update 0, deleted 0
Unit test platform: [tizen]
UT result: Pass:7, Fail 1, block 1
Fail: Crosswalk_AudioPolicy_WebappAudioPlay: two webapp audio can be play at the same time
Block:Crosswalk_AudioPolicy_Muliple_Sinks: No bluetooth headset
[doc] Transfer pdf to md in doc directory
Merge pull request #934 from kangxu/kangxu/wrt/doc/addMD

[doc] Transfer pdf to md in doc directory
Merge pull request #933 from haoxli/haoxli/webapi/xwalk-2561-video

[webapi][xwalk-2561] Update codes based on Chromium implement
Merge pull request #931 from linshen/shenlin/addaudioplicy

[wrt][XWALK-1668]Add new audiopolicy testcase
[usecase-webapi] Demo for adding pluggable step info

Impacted TCs num(approve): New 0, Update 0, Delete 1
Unit test Platform: Andriod
Unit test result summary: Pass 0, Fail 0, Blocked 0
Merge pull request #896 from JianfengXu/xujf/usecase/step

[usecase-webapi] Demo for adding pluggable step info