Add GN variables for controlling header checking.

tools/gn/target.h

Index: tools/gn/target.h
diff --git a/tools/gn/target.h b/tools/gn/target.h
index 9b7ff6d10201778c13e88dbe0b4c14bd1f0d4775..aff7706cbe71055974e399a2fc39744a0d07181a 100644
--- a/tools/gn/target.h
+++ b/tools/gn/target.h
@@ -91,6 +91,10 @@ class Target : public Item {
   const FileList& public_headers() const { return public_headers_; }
   FileList& public_headers() { return public_headers_; }
 
+  // Whether this target's includes should be checked by "gn check".
+  bool check_includes() const { return check_includes_; }
+  void set_check_includes(bool ci) { check_includes_ = ci; }
+
   // Compile-time extra dependencies.
   const FileList& inputs() const { return inputs_; }
   FileList& inputs() { return inputs_; }
@@ -148,6 +152,14 @@ class Target : public Item {
     return forward_dependent_configs_;
   }
 
+  // Dependencies that can include files from this target.
+  const std::set<Label>& allow_circular_includes_from() const {
+    return allow_circular_includes_from_;
+  }
+  std::set<Label>& allow_circular_includes_from() {
+    return allow_circular_includes_from_;
+  }
+
   const UniqueVector<const Target*>& inherited_libraries() const {
     return inherited_libraries_;
   }
@@ -217,6 +229,7 @@ class Target : public Item {
   FileList sources_;
   bool all_headers_public_;
   FileList public_headers_;
+  bool check_includes_;
   FileList inputs_;
   FileList data_;
 
@@ -241,6 +254,8 @@ class Target : public Item {
   UniqueVector<LabelConfigPair> direct_dependent_configs_;
   UniqueVector<LabelTargetPair> forward_dependent_configs_;
 
+  std::set<Label> allow_circular_includes_from_;
+
   bool external_;
 
   // Static libraries and source sets from transitive deps. These things need