Make existing unit tests only run on GL contexts

tests/ReadWriteAlphaTest.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Test.h"
 
 // This test is specific to the GPU backend.
@@ skipping 15 matching lines @@
             if (e != a) {
                 ERRORF(reporter,
                        "Failed alpha readback. Expected: 0x%02x, Got: 0x%02x at (%d,%d), %s",
                        e, a, x, y, extraMsg.c_str());
                 return;
             }
         }
     }
 }
 
-DEF_GPUTEST_FOR_RENDERING_CONTEXTS(ReadWriteAlpha, reporter, ctxInfo) {
+DEF_GPUTEST_FOR_GL_RENDERING_CONTEXTS(ReadWriteAlpha, reporter, ctxInfo) {
     unsigned char alphaData[X_SIZE * Y_SIZE];
 
     bool match;
     static const size_t kRowBytes[] = {0, X_SIZE, X_SIZE + 1, 2 * X_SIZE - 1};
     for (int rt = 0; rt < 2; ++rt) {
         GrSurfaceDesc desc;
         // let Skia know we will be using this texture as a render target
         desc.fFlags     = rt ? kRenderTarget_GrSurfaceFlag : kNone_GrSurfaceFlags;
         // it is a single channel texture
         desc.fConfig    = kAlpha_8_GrPixelConfig;
@@ skipping 129 matching lines @@
                 SkString msg;
                 msg.printf("rt:%d, rb:%d", rt, SkToU32(rowBytes));
                 validate_alpha_data(reporter, X_SIZE, Y_SIZE, readback.get(), nonZeroRowBytes,
                                     alphaData, msg);
             }
         }
     }
 }
 
 #endif