GN: make SkPMColor BGRA on Linux. All other platforms are auto-detected.

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 55 matching lines @@
     "SK_HAS_PNG_LIBRARY",
     "SK_HAS_WEBP_LIBRARY",
 
     # TODO(halcanary): make this the default; this is the value Android uses.
     "SK_SFNTLY_SUBSETTER=\"sample/chromium/font_subsetter.h\"",
 
     "TURBO_HAS_565",
     "TURBO_HAS_CROP",
     "TURBO_HAS_SKIP",
   ]
+  if (is_linux) {
+    defines += [ "SK_SAMPLES_FOR_X" ]
+  }
 }
 
 # Any code that's linked into Skia-the-library should use this config via += skia_library_configs.
 config("skia_library") {
   visibility = [ ":*" ]
   defines = [ "SKIA_IMPLEMENTATION=1" ]
 }
 
 skia_library_configs = [
   ":skia_public",
@@ skipping 494 matching lines @@
       "tools/SkShaper_harfbuzz.cpp",
       "tools/using_skia_and_harfbuzz.cpp",
     ]
     deps = [
       ":skia",
       "//third_party/harfbuzz",
     ]
     testonly = true
   }
 }