[ios] Disable new build system in Xcode 10 for convert_gn_xcodeproj.

Xcode 10 obscures compile failures when build output includes relative paths,
and our xctest-build-sources fail to compile.  Disable build system for now.

Bug: 852522
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ice0e5c7e6734270f804ae0f8a21cf5621ff13f58
Reviewed-on: https://chromium-review.googlesource.com/1182143Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584722}

ios/build/tools/convert_gn_xcodeproj.py

@@ -145,6 +145,18 @@ def UpdateProductsProject(file_input, file_output, configurations):
   WriteXcodeProject(file_output, json.dumps(json_data))
 
 
+def DisableNewBuildSystem(output_dir):
+  """Disables the new build system due to crbug.com/852522 """
+  xcwspacesharedsettings = os.path.join(output_dir, 'all.xcworkspace',
+      'xcshareddata', 'WorkspaceSettings.xcsettings')
+  if os.path.isfile(xcwspacesharedsettings):
+    json_data = json.loads(LoadXcodeProjectAsJSON(xcwspacesharedsettings))
+  else:
+    json_data = {}
+  json_data['BuildSystemType'] = 'Original'
+  WriteXcodeProject(xcwspacesharedsettings, json.dumps(json_data))
+
+
 def ConvertGnXcodeProject(input_dir, output_dir, configurations):
   '''Tweak the Xcode project generated by gn to support multiple configurations.
 
@@ -171,6 +183,11 @@ def ConvertGnXcodeProject(input_dir, output_dir, configurations):
   xcwspace = os.path.join('all.xcworkspace', 'contents.xcworkspacedata')
   CopyFileIfChanged(os.path.join(input_dir, xcwspace),
                     os.path.join(output_dir, xcwspace))
+
+
+  # TODO(crbug.com/852522): Disable new BuildSystemType.
+  DisableNewBuildSystem(output_dir)
+
   # TODO(crbug.com/679110): gn has been modified to remove 'sources.xcodeproj'
   # and keep 'all.xcworkspace' and 'products.xcodeproj'. The following code is
   # here to support both old and new projects setup and will be removed once gn