GN: get echo-headers sources via exec_script

gyp/skiaserve.gyp

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # GYP file to build performance testbench.
 #
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
     {
       'target_name': 'skiaserve',
       'type': 'executable',
       'include_dirs': [
         '../src/core',
         '../src/gpu',
         '../src/ports',
         '../tools/debugger',
         '../tools/json',
       ],
-      'sources': [ 
+      'sources': [
         # Stuff for the debug canvas
         '../tools/debugger/SkDrawCommand.h',
         '../tools/debugger/SkDrawCommand.cpp',
         '../tools/debugger/SkDebugCanvas.h',
         '../tools/debugger/SkDebugCanvas.cpp',
         '../tools/debugger/SkJsonWriteBuffer.h',
         '../tools/debugger/SkJsonWriteBuffer.cpp',
         '../tools/debugger/SkObjectParser.h',
         '../tools/debugger/SkObjectParser.cpp',
         '../tools/debugger/SkOverdrawMode.h',
         '../tools/debugger/SkOverdrawMode.cpp',
-        '<!@(python find.py ../tools/skiaserve "*.cpp")',
-        '<!@(python find.py ../tools/skiaserve/urlhandlers "*.cpp")',
+        '<!@(python find.py "*.cpp" ../tools/skiaserve ../tools/skiaserve/urlhandlers)',
       ],
       'dependencies': [
         'flags.gyp:flags',
         'gputest.gyp:skgputest',
         'jsoncpp.gyp:jsoncpp',
         'libpng.gyp:libpng',
         'microhttpd.gyp:microhttpd',
         'skia_lib.gyp:skia_lib',
         'tools.gyp:crash_handler',
         'tools.gyp:picture_utils',
         'tools.gyp:proc_stats',
         'tools.gyp:resources',
         'tools.gyp:url_data_manager',
       ],
     },
   ],
 }