Clean up MipmapTest This refactoring is done to prepare for adding more mipmap tests for ES3 where base level / max level is changed from 0. 1. Reuse drawQuad() from ANGLETest instead of having completely custom quad drawing code, and reuse the same clear+draw function for both 2D and cube map texture tests. 2. Add a helper function for clearing texture level 0 with glClear. 3. Remove a few redundant texture binding and useProgram calls. 4. Rely on sampler uniforms being set to 0 by default and texture unit 0 being active by default. 5. Remove "Offscreen" from variable names. 6. Split SetUp functions to helper functions. 7. Reuse FillWithRGBA from TextureTest also in MipmapTest. BUG=angleproject:596 TEST=angle_end2end_tests Change-Id: I9fd738f4b8b0a19c7aa1d267f7dbaa40a6935631 Reviewed-on: https://chromium-review.googlesource.com/338791 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>