Remove gpu shader optimatization for solid white or trans black colors

Running test on the added bench which draws a grid of all white paths, all blue paths, or alternating checkered white/blue paths.

With optimization in (ms):
         White       Blue        Checkered
Linux    ~80         ~80         ~160
N7       ~800        ~1100       ~1500
Moto-e   ~830        ~1100       ~2500

Without optimization in (ms):
         White       Blue        Checkered
Linux    ~80         ~80         ~80
N7       ~1100       ~1100       ~1100
Moto-e   ~1100       ~1100       ~1500

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/5f78d2251a440443c9eaa321dad058d7a32bfef7

R=bsalomon@google.com

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/375823005
diff --git a/tests/GLProgramsTest.cpp b/tests/GLProgramsTest.cpp
index 899b42f..9da1ecc 100644
--- a/tests/GLProgramsTest.cpp
+++ b/tests/GLProgramsTest.cpp
@@ -73,8 +73,9 @@
     // if the effects have used up all off the available attributes,
     // don't try to use color or coverage attributes as input
     do {
-        header->fColorInput = static_cast<GrGLProgramDesc::ColorInput>(
-                                  random->nextULessThan(kColorInputCnt));
+        uint32_t colorRand = random->nextULessThan(2);
+        header->fColorInput = (0 == colorRand) ? GrGLProgramDesc::kAttribute_ColorInput :
+                                                 GrGLProgramDesc::kUniform_ColorInput;
     } while (GrDrawState::kMaxVertexAttribCnt <= currAttribIndex &&
              kAttribute_ColorInput == header->fColorInput);