Add GN variables for controlling header checking.

tools/gn/variables.h

Index: tools/gn/variables.h
diff --git a/tools/gn/variables.h b/tools/gn/variables.h
index e2fd2546a2bcca76420be4ed98422a52f30e3d0a..830a05f2b39147c0eecc9dd26575f7bb1b8424f3 100644
--- a/tools/gn/variables.h
+++ b/tools/gn/variables.h
@@ -71,6 +71,10 @@ extern const char kAllDependentConfigs[];
 extern const char kAllDependentConfigs_HelpShort[];
 extern const char kAllDependentConfigs_Help[];
 
+extern const char kAllowCircularIncludesFrom[];
+extern const char kAllowCircularIncludesFrom_HelpShort[];
+extern const char kAllowCircularIncludesFrom_Help[];
+
 extern const char kArgs[];
 extern const char kArgs_HelpShort[];
 extern const char kArgs_Help[];
@@ -95,6 +99,10 @@ extern const char kCflagsObjCC[];
 extern const char kCflagsObjCC_HelpShort[];
 extern const char* kCflagsObjCC_Help;
 
+extern const char kCheckIncludes[];
+extern const char kCheckIncludes_HelpShort[];
+extern const char kCheckIncludes_Help[];
+
 extern const char kConfigs[];
 extern const char kConfigs_HelpShort[];
 extern const char kConfigs_Help[];