Reduce absolute paths in generated ninja files

ppapi/native_client/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/nacl/config.gni")
 
 if (is_nacl) {
   group("ppapi_lib") {
     deps = [
@@ skipping 77 matching lines @@
     sources = [
       irt_debug,
       irt_stripped,
     ]
     outputs = [
       irt_final,
     ]
 
     script = "irt_debuglink.py"
     args = [
-      rebase_path(objcopy),
-      rebase_path(irt_debug),
-      rebase_path(irt_stripped),
-      rebase_path(irt_final),
+      rebase_path(objcopy, root_build_dir),
+      rebase_path(irt_debug, root_build_dir),
+      rebase_path(irt_stripped, root_build_dir),
+      rebase_path(irt_final, root_build_dir),
     ]
   }
 }
 
 group("irt") {
   # The nacl_irt needs to be marked as data dependency so that it's
   # properly resolved as runtime dependency. However, some target
   # use nacl_irt as input for actions which require it to be marked
   # as public dependency as well.
   irt_deps = [
     ":nacl_irt_debug(//build/toolchain/nacl:irt_${target_cpu})",
     ":nacl_irt_debuglink(//build/toolchain/nacl:irt_${target_cpu})",
   ]
   public_deps = irt_deps
   data_deps = irt_deps
 
   # On 32-bit Windows, we also ship the 64-bit IRT.
   if (is_win && target_cpu == "x86") {
     irt_x64_deps = [
       ":nacl_irt_debug(//build/toolchain/nacl:irt_x64)",
       ":nacl_irt_debuglink(//build/toolchain/nacl:irt_x64)",
     ]
     public_deps += irt_x64_deps
     data_deps += irt_x64_deps
   }
 }