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/src/gpu/GrDrawState.cpp b/src/gpu/GrDrawState.cpp
index 14ba6fa..02c7920 100644
--- a/src/gpu/GrDrawState.cpp
+++ b/src/gpu/GrDrawState.cpp
@@ -409,6 +409,15 @@
     return kNone_BlendOpt;
 }
 
+bool GrDrawState::canIgnoreColorAttribute() const {
+    if (fBlendOptFlags & kInvalid_BlendOptFlag) {
+        this->getBlendOpts();
+    }
+    return SkToBool(fBlendOptFlags & (GrDrawState::kEmitTransBlack_BlendOptFlag |
+                                      GrDrawState::kEmitCoverage_BlendOptFlag));
+}
+
+
 ////////////////////////////////////////////////////////////////////////////////
 
 void GrDrawState::AutoViewMatrixRestore::restore() {