Add GN conversions for more GYP flags

tools/gn/secondary/skia/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 enable_printing = true  # TODO(brettw) pass this feature flag in somehow.
 skia_support_gpu = !is_ios
 skia_support_pdf = !is_ios && enable_printing
 
 # External-facing config for dependent code.
 config("skia_config") {
@@ skipping 22 matching lines @@
     "GR_GL_CUSTOM_SETUP_HEADER=\"GrGLConfig_chrome.h\"",
   ]
 
   if (skia_support_gpu) {
     include_dirs += [
       "//third_party/skia/include/gpu",
       "//third_party/skia/src/gpu",
     ]
     defines += "SK_SUPPORT_GPU=1"
   } else {
-    defines = "SK_SUPPORT_GPU=0"
+    defines += "SK_SUPPORT_GPU=0"
   }
 
   # For POSIX platforms, prefer the Mutex implementation provided by Skia
   # since it does not generate static initializers.
   if (is_posix) {
     defines += "SK_USE_POSIX_THREADS"
   }
 
   if (is_android) {
     defines += [
@@ skipping 1279 matching lines @@
     ":skia_config",
     "//build/config/compiler:no_chromium_code"
   ]
 
   if (is_linux) {
     cflags = [ "-msse3" ]
   } else if (is_mac) {
     cflags = [ "-mssse3" ] # Note the third 's'.
   }
 }