Add functionality to find xctest files under application target.

tools/gn/xcode_writer.h

Index: tools/gn/xcode_writer.h
diff --git a/tools/gn/xcode_writer.h b/tools/gn/xcode_writer.h
index 7373cff04e5ac41bdd04e7637a05d11162b29cd3..2280b7c21cb319df614d3f8e3c79de1054305301 100644
--- a/tools/gn/xcode_writer.h
+++ b/tools/gn/xcode_writer.h
@@ -60,6 +60,11 @@ class XcodeWriter {
                             std::vector<const Target*>* targets,
                             Err* err);
 
+  // Filters list of targets to only return ones that are application bundles.
+  static void FilterApplicationTargets(
+      const std::vector<const Target*>& targets,
+      std::vector<const Target*>* application_targets);
+
   // Generate the "products.xcodeproj" project that reference all products
   // (i.e. targets that have a build artefact usable from Xcode, mostly
   // application bundles).