x11: Split the gfx_x11 target into its own gyp file.

ui/gfx/x/gfx_x11.gyp

+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+{
+  'variables': {
+    'chromium_code': 1,
+    'use_x11': 1,  # It is necessary to explicitly set use_x11 here to make sure
+                   # that the exclusion rules in filename_rules.gypi do not
+                   # exclude the x11* files.
+  },
+
+  'targets': [
+    {
+      'target_name': 'gfx_x11',
+      'type': '<(component)',
+      'dependencies': [
+        '../../../base/base.gyp:base',
+        '../../../build/linux/system.gyp:x11',
+      ],
+      'defines': [
+        'GFX_IMPLEMENTATION',
+      ],
+      'sources': [
+        'x11_atom_cache.cc',
+        'x11_atom_cache.h',
+        'x11_connection.cc',
+        'x11_connection.h',
+        'x11_error_tracker.cc',
+        'x11_error_tracker.h',
+        'x11_types.cc',
+        'x11_types.h',
+      ],
+    },
+  ]
+}