Fix include directories in Windows GN build.

build/json_schema_api.gni

Index: build/json_schema_api.gni
diff --git a/build/json_schema_api.gni b/build/json_schema_api.gni
index dd8ab922f3f705d22f8b7337436ad2294cc05930..4811968a76cf12f643872e77defe5afd573bc31e 100644
--- a/build/json_schema_api.gni
+++ b/build/json_schema_api.gni
@@ -71,7 +71,7 @@ template("json_schema_api") {
 
   generated_config_name = target_name + "_generated_config"
   config(generated_config_name) {
-    include_dirs = [ target_gen_dir ]

[Slava Chigrin, 2015/01/13 15:46:05]

Here target_gen_dir is something like //out/gnRel/gen/extensions/common/api
and  root_gen_dir is like
//out/gnRel/gen

We must NOT add target_gen_dir  to the include path, since there is file
windows.h in directory //out/gnRel/gen/extensions/common/api . That causes
conflicts with system header with the same name, producing a lot of weird
compile errors on Windows.

Generated file included in chrome by more specific path, like
#include "chrome/common/extensions/api/windows.h" , so everyting should be OK
after that fix.

+    include_dirs = [ root_gen_dir ]
     visibility = target_visibility
   }