[Android] Add DEPS for khronos's OpenCL header files to support WebCL.

It is a dependent patch for WebCL support.
This subversion repo comes from khronos, which is the MIT license.
It hosts the OpenCL header files, and its version is 1.2,
which is the latest version supported on Android platform.

BUG=XWALK-2359
Merge pull request #2426 from junmin-zhu/header

[Android] Add DEPS for khronos's OpenCL header files to support WebCL.