GN: define fake-gamma configuration to match our bots.

BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
 }
 
 skia_public_includes = [
   "include/android",
@@ skipping 40 matching lines @@
     "src/pathops",
     "src/pdf",
     "src/ports",
     "src/sfnt",
     "src/utils",
     "third_party/etc1",
     "third_party/ktx",
   ]
 
   defines = [
+    "SK_GAMMA_SRGB",
+    "SK_GAMMA_APPLY_TO_A8",
+
     "SK_HAS_GIF_LIBRARY",
     "SK_HAS_JPEG_LIBRARY",
     "SK_HAS_PNG_LIBRARY",
     "SK_HAS_WEBP_LIBRARY",
   ]
 }
 
 # Any code that's linked into Skia-the-library should use this config via += skia_library_configs.
 config("skia_library") {
   visibility = [ ":*" ]
@@ skipping 401 matching lines @@
       ":flags",
       ":gm",
       ":gpu_tool_utils",
       ":skia",
       ":tool_utils",
       "//third_party/jsoncpp",
     ]
     testonly = true
   }
 }