[WebRTC] Add libwebrtc build infrastructure

[WebRTC] Add libwebrtc build infrastructure
https://bugs.webkit.org/show_bug.cgi?id=167207

Patch by Youenn Fablet <youenn@apple.com> on 2017-01-20
Reviewed by Alex Christensen.

.:

* WebKit.xcworkspace/contents.xcworkspacedata:

Source/ThirdParty:

Checking-in files used for building libwebrtc.
The principle is the following:
- Select a revision of libwebrtc
- Ask libwebrtc build system to generate the project.json (something like gn gen --ide json ...)
- Copy project.json here and the libwebrtc selected revision in a libwebrtc/libwebrtc folder.
- Call Tools/Scripts/generate-libwebrtc-cmake which will update CMakeLists.txt
- Build libwebrtc with CMake

* libwebrtc/CMakeLists.txt: Added (Generated by Tools/Scripts/generate-libwebrtc-cmake using project.json as input).
* libwebrtc/Makefile: Added.
* libwebrtc/WebKit/project.json: Added (Generated by libwebrtc build system).
* libwebrtc/WebKit/rtc_sdk_framework_objc_info_plist.plist: Added.
* libwebrtc/libwebrtc.xcodeproj/project.pbxproj: Added.

Tools:

Adding a script doing the libwebrtc project json to CMake file conversion.

* Scripts/generate-libwebrtc-cmake: Added.
* Scripts/webkitpy/libwebrtc/__init__.py: Added.
* Scripts/webkitpy/libwebrtc/generate_cmake.py: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@210973 268f45cc-cd09-0410-ab3c-d52691b4dbfc