Add stress test for WebCLSampler

Add stress test for WebCLSampler.

        Reviewed by NOBODY (OOPS!).

        * webcl/tests/webcl-stress-sampler-expected.txt: Added.
        * webcl/tests/webcl-stress-sampler.html: Added.
        * webcl/resources/webcl-test.js: Change the initCLContext to use deviceType
        passed in initCL.