Port "Eclipse CDT settings" file generation from GYP to GN

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 32 matching lines @@
         'config_values.cc',
         'config_values.h',
         'config_values_extractors.cc',
         'config_values_extractors.h',
         'config_values_generator.cc',
         'config_values_generator.h',
         'copy_target_generator.cc',
         'copy_target_generator.h',
         'deps_iterator.cc',
         'deps_iterator.h',
+        'eclipse_writer.cc',
+        'eclipse_writer.h',
         'err.cc',
         'err.h',
         'escape.cc',
         'escape.h',
         'exec_process.cc',
         'exec_process.h',
         'filesystem_utils.cc',
         'filesystem_utils.h',
         'function_exec_script.cc',
         'function_foreach.cc',
@@ skipping 223 matching lines @@
           'dependencies': [
             'gn_unittests',
           ],
           'includes': [ '../../build/isolate.gypi' ],
           'sources': [ 'gn_unittests.isolate' ],
         },
       ],
     }],
   ],
 }