WebUI generator GN build file fixed.

In case when GN output dir is inside src/, GN uses pathes relative to src/
directory that start with // (//out/Release/gen). But python interprets such
pathes as relative to /.

BUG=459230

Review URL: https://codereview.chromium.org/973873003

Cr-Commit-Position: refs/heads/master@{#319058}

components/webui_generator/generator/wug.gni

@@ -41,7 +41,7 @@ template("wug") {
   helper_args = [
     rebase_path(declaration_path, root_build_dir),
     "--destination",
-    out_dir,
+    rebase_path(out_dir, root_build_dir),
     "--root",
     src_root,
     "--gn",
@@ -85,7 +85,7 @@ template("wug") {
       "--root",
       src_root,
       "--destination",
-      out_dir,
+      rebase_path(out_dir, root_build_dir),
     ]
   }