Add a stress test for WebCLBuffer

Add a stress test for WebCLBuffer.

        Reviewed by NOBODY (OOPS!).

        A WebCLBuffer is created several times with different alloc sizes. The
        maximum size is delimited by DEVICE_MAX_MEM_ALLOC_SIZE.
        The variable STRESS_NUM_LOOPS (in webcl-test-utils.js) controls how many
        WebCLBuffers will be created in the test. This variable will be used by
        the others stress tests also.

        * webcl/resources/webcl-test-utils.js:
        * webcl/tests/webcl-stress-buffer-expected.txt: Added.
        * webcl/tests/webcl-stress-buffer.html: Added.