GN: polyfill is_fuchsia

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 API, generally provided by :skia.
 config("skia_public") {
@@ skipping 52 matching lines @@
 
   defines = [ "SKIA_IMPLEMENTATION=1" ]
 }
 
 skia_library_configs = [
   ":skia_public",
   ":skia_private",
   ":skia_library",
 ]
 
+if (!defined(is_fuchsia)) {
+  is_fuchsia = current_os == "fuchsia"
+}
+
 unwanted_configs = []
 
 if (!is_fuchsia) {
   # Chrome's GN environment is mostly helpful, but a couple default configs tend to get in the way.
   unwanted_configs += [
     "//build/config/clang:find_bad_constructs",  # Chrome style checks.
     "//build/config:feature_flags",  # A bunch of #defines we don't care about.
   ]
 }
 
@@ skipping 155 matching lines @@
   if (is_mac) {
     sources += [
       "src/ports/SkFontHost_mac.cpp",
       "src/ports/SkImageEncoder_CG.cpp",
       "src/ports/SkImageGeneratorCG.cpp",
     ]
     libs += [ "ApplicationServices.framework" ]
   }
 
   if (is_fuchsia) {
-    sources += [
-      "src/ports/SkFontMgr_empty_factory.cpp",
-    ]
+    sources += [ "src/ports/SkFontMgr_empty_factory.cpp" ]
   }
 }
 
 executable("example") {
   configs -= unwanted_configs
 
   sources = [
     "cmake/example.cpp",
   ]
   deps = [
     ":skia",
   ]
 
   libs = []
   if (is_mac) {
     libs += [ "OpenGL.framework" ]
   }
 }