GN: get echo-headers sources via exec_script

gn/echo_headers.py

 #!/usr/bin/env python
 #
 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import sys
-import os
-import os.path
 
-blacklist = [
-  'GrGLConfig_chrome.h',
-  'SkFontMgr_fontconfig.h',
-]
-
-headers = []
-for d in sys.argv[2:]:
-  headers.extend([f for f in os.listdir(d)
-                  if os.path.isfile(os.path.join(d,f)) and f.endswith('.h')])
 with open(sys.argv[1], "w") as f:
   f.write('// skia.h generated by GN.\n')
   f.write('#ifndef skia_h_DEFINED\n')
   f.write('#define skia_h_DEFINED\n')
-  for h in headers:
-    if h not in blacklist:
-      f.write('#include "' + h + '"\n')
+  for h in sys.argv[2:]:
+    f.write('#include "' + h + '"\n')
   f.write('#endif//skia_h_DEFINED\n')