tools: Fix extensions path in check_grd_for_unused_strings.py

At some point extensions strings moved into extensions/strings.

BUG=none
TEST=runs

Review-Url: https://codereview.chromium.org/2802723005
Cr-Commit-Position: refs/heads/master@{#462586}
diff --git a/tools/check_grd_for_unused_strings.py b/tools/check_grd_for_unused_strings.py
index 8124b576..21afc8a 100755
--- a/tools/check_grd_for_unused_strings.py
+++ b/tools/check_grd_for_unused_strings.py
@@ -152,7 +152,7 @@
       os.path.join(chrome_dir, 'renderer', 'resources',
                    'renderer_resources.grd'),
       os.path.join(device_base_dir, 'bluetooth', 'bluetooth_strings.grd'),
-      os.path.join(src_dir, 'extensions', 'extensions_strings.grd'),
+      os.path.join(src_dir, 'extensions', 'strings', 'extensions_strings.grd'),
       os.path.join(src_dir, 'ui', 'resources', 'ui_resources.grd'),
       os.path.join(src_dir, 'ui', 'webui', 'resources', 'webui_resources.grd'),
       os.path.join(ui_strings_dir, 'app_locale_settings.grd'),