clean up default precision handling

src/gpu/gl/GrGLShaderVar.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLShaderVar_DEFINED
 #define GrGLShaderVar_DEFINED
 
@@ skipping 189 matching lines @@
     static const char* PrecisionString(Precision p, GrGLStandard standard) {
         // Desktop GLSL has added precision qualifiers but they don't do anything.
         if (kGLES_GrGLStandard == standard) {
             switch (p) {
                 case kLow_Precision:
                     return "lowp ";
                 case kMedium_Precision:
                     return "mediump ";
                 case kHigh_Precision:
                     return "highp ";
-                case kDefault_Precision:
-                    return "";
                 default:
                     SkFAIL("Unexpected precision type.");
             }
         }
         return "";
     }
 
 private:
     static const char* TypeModifierString(TypeModifier t, GrGLSLGeneration gen) {
         switch (t) {
@@ skipping 21 matching lines @@
 
     Origin          fOrigin;
     /// Work around driver bugs on some hardware that don't correctly
     /// support uniform float []
     bool            fUseUniformFloatArrays;
 
     typedef GrShaderVar INHERITED;
 };
 
 #endif