Fix unit test crash caused by OSMesa not supporting rendering to GL_ALPHA8

src/gpu/gl/GrGLUtil.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLUtil_DEFINED
 #define GrGLUtil_DEFINED
 
@@ skipping 33 matching lines @@
     kOther_GrGLVendor
 };
 
 enum GrGLRenderer {
     kTegra2_GrGLRenderer,
     kTegra3_GrGLRenderer,
     kPowerVR54x_GrGLRenderer,
     kPowerVRRogue_GrGLRenderer,
     kAdreno3xx_GrGLRenderer,
     kAdreno4xx_GrGLRenderer,
+    kOSMesa_GrGLRenderer,
     kOther_GrGLRenderer
 };
 
 enum GrGLDriver {
     kMesa_GrGLDriver,
     kChromium_GrGLDriver,
     kNVIDIA_GrGLDriver,
     kIntel_GrGLDriver,
     kANGLE_GrGLDriver,
     kUnknown_GrGLDriver
@@ skipping 137 matching lines @@
         GR_GL_LOG_CALLS_IMPL(X);                                \
     } while (false)
 
 // call glGetError without doing a redundant error check or logging.
 #define GR_GL_GET_ERROR(IFACE) (IFACE)->fFunctions.fGetError()
 
 GrGLenum GrToGLStencilFunc(GrStencilFunc basicFunc);
 
 
 #endif