Clean up specifying colors and texture scale in MipmapTest

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/344513
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/tests/test_utils/ANGLETest.cpp b/src/tests/test_utils/ANGLETest.cpp
index 183c37b..c91dd60 100644
--- a/src/tests/test_utils/ANGLETest.cpp
+++ b/src/tests/test_utils/ANGLETest.cpp
@@ -20,6 +20,7 @@
 const GLColor GLColor::blue  = GLColor(0u, 0u, 255u, 255u);
 const GLColor GLColor::cyan  = GLColor(0u, 255u, 255u, 255u);
 const GLColor GLColor::black = GLColor(0u, 0u, 0u, 255u);
+const GLColor GLColor::white = GLColor(255u, 255u, 255u, 255u);
 
 namespace
 {