Fix for disabled color writes xp on n6

src/gpu/effects/GrDisableColorXP.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #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:
     GrGLDisableColorXP(const GrProcessor&) {}
 
     ~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 {}
 
     static void GenKey(const GrProcessor&, const GrGLCaps&, GrProcessorKeyBuilder*) {}
 
 private:
     typedef GrGLXferProcessor INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
@@ skipping 27 matching lines @@
 
 GR_DEFINE_XP_FACTORY_TEST(GrDisableColorXPFactory);
 
 GrXPFactory* GrDisableColorXPFactory::TestCreate(SkRandom* random,
                                                   GrContext*,
                                                   const GrDrawTargetCaps&,
                                                   GrTexture*[]) {
     return GrDisableColorXPFactory::Create();
 }