Remove unnecessary scale parameter from MipmapTestES3, and use the
GLColor class and EXPECT_PIXEL_COLOR_EQ instead of EXPECT_PIXEL_EQ for
greater readability.

BUG=angleproject:596
TEST=angle_end2end_tests

Change-Id: I79c30ce85be5d554d89197f8f1ce7ab0c51c11b6
Reviewed-on: https://chromium-review.googlesource.com/344513Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>