[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.

BUG=XWALK-2550
same comment as `WebCLBuffer::create()`