Fix for disabled color writes xp on n6

src/gpu/effects/GrDisableColorXP.cpp

Index: src/gpu/effects/GrDisableColorXP.cpp
diff --git a/src/gpu/effects/GrDisableColorXP.cpp b/src/gpu/effects/GrDisableColorXP.cpp
index ba6a45b7809735164c3f922d7e16194699762b6a..93833851921d3c6eadfa77825b635e4cb5317dc8 100644
--- a/src/gpu/effects/GrDisableColorXP.cpp
+++ b/src/gpu/effects/GrDisableColorXP.cpp
@@ -8,6 +8,8 @@
 #include "effects/GrDisableColorXP.h"
 #include "GrProcessor.h"
 #include "gl/GrGLXferProcessor.h"
+#include "gl/builders/GrGLFragmentShaderBuilder.h"
+#include "gl/builders/GrGLProgramBuilder.h"
 
 class GrGLDisableColorXP : public GrGLXferProcessor {
 public:
@@ -15,7 +17,13 @@ public:
 
     ~GrGLDisableColorXP() SK_OVERRIDE {}
 
-    void emitCode(const EmitArgs& args) SK_OVERRIDE {}
+    void emitCode(const EmitArgs& args) SK_OVERRIDE {
+        // This emit code should be empty. However, on the nexus 6 there is a driver bug where if
+        // you do not give gl_FragColor a value, the gl context is lost and we end up drawing
+        // nothing. So this fix just sets the gl_FragColor arbitrarily to 0.
+        GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+        fsBuilder->codeAppendf("%s = vec4(0);", args.fOutputPrimary);
+    }
 
     void setData(const GrGLProgramDataManager&, const GrXferProcessor&) SK_OVERRIDE {}