Reduce absolute paths in generated ninja files

extensions/BUILD.gn

 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//extensions/features/features.gni")
 import("//testing/test.gni")
 import("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 assert(enable_extensions)
 
@@ skipping 17 matching lines @@
   source = "browser/resources/extensions_browser_resources.grd"
   outputs = [
     "grit/extensions_browser_resources.h",
     "grit/extensions_browser_resources_map.cc",
     "grit/extensions_browser_resources_map.h",
     "extensions_browser_resources_100_percent.pak",
     "extensions_browser_resources_200_percent.pak",
   ]
   grit_flags = [
     "-E",
-    "mojom_root=" + rebase_path(root_gen_dir),
+    "mojom_root=" + rebase_path(root_gen_dir, root_build_dir),
   ]
 }
 
 grit("extensions_renderer_resources") {
   source = "renderer/resources/extensions_renderer_resources.grd"
   outputs = [
     "grit/extensions_renderer_resources.h",
     "extensions_renderer_resources.pak",
   ]
   grit_flags = [
     "-E",
-    "mojom_root=" + rebase_path(root_gen_dir),
+    "mojom_root=" + rebase_path(root_gen_dir, root_build_dir),
   ]
 
   deps = [
     "//device/serial:serial_mojo__generator",
     "//extensions/common:mojo__generator",
     "//extensions/common/api:mojom__generator",
   ]
 }
 
 static_library("test_support") {
@@ skipping 294 matching lines @@
     "//testing/gtest",
     "//third_party/hunspell",
     "//third_party/icu",
     "//third_party/libpng",
     "//third_party/zlib",
     "//ui/base:test_support",
     "//ui/resources:ui_test_pak",
     "//ui/web_dialogs:test_support",
   ]
 }