add antialias coverage for conformance/context/premultiplyalpha-test.html.
Remove duplicated glEnumToString functionality in webgl-test-utils
Remove unused webglTestLog function
Remove duplicate of glErrorShouldBe and always use one in wtu
Remove duplicate of shouldGenerateGLError and always use one in wtu

In addition, shouldGenerateGLError now reuses glErrorShouldBe. Error
message format changes a bit, but contains the same information.
Get rid of unused webgl-test.js entirely
Merge pull request #407 from JunJiang/add-antialiasing-case

add antialias coverage for conformance/context/premultiplyalpha-test.html
Merge pull request #410 from NVIDIA/utility-cleanup

Utility cleanup