[Android] WebCL support for Crosswalk.

WebCL 1.0 defines a JavaScript binding to the Khronos OpenCL standard
for heterogeneous parallel computing. Its spec is available at:
http://www.khronos.org/registry/webcl/specs/latest/1.0/

This patch includes WebCL basic APIs support, dynamic loading mechanism
etc. WebCL support is controlled by "ENABLE_WEBCL" flag.
Currently, WebCL is disabled by default until the other codes are ready,
for example, the gl and cl memory sharing and security check etc.

If you want to play with WebCL, please add "-Duse_webcl=1" when running
the "xwalk/gyp_xwalk" before building. And run conformance test
(https://github.com/KhronosGroup/WebCL-conformance) in the XWalkCoreShell.

BUG=XWALK-2550
above var document and page are not necessary.
```
if (!m_window.document() || !m_window.document()->page())
    return nullptr;
```