WebUI: Prevent request lists from colliding for hosts building twice

Currently, the request list path in optimize_webui is computed based
only on the host name. Add a '-v2' to the path for Polymer 2 builds
(only applies to OS settings), to avoid the possibility of v2 and v3
builds accessing the same request list when generating dependency files.

Collisions result in v2 dependencies being listed in the v3 build.d
file, and vice versa, leading to build failures.

Bug: 1132403
Change-Id: I4d92676db501669ddb99b204abfc4413dd8e2087
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491241
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819755}

chrome/browser/resources/optimize_webui.py

@@ -347,6 +347,8 @@ def _optimize(in_folder, args):
       bundled_paths = _bundle_v3(tmp_out_dir, in_path, out_path,
                                  manifest_out_path, args, excludes)
     else:
+      # Ensure Polymer 2 and Polymer 3 request lists don't collide.
+      manifest_out_path = _request_list_path(out_path, args.host + '-v2')
       pcb_out_paths = [os.path.join(out_path, f) for f in args.html_out_files]
       bundled_paths = _bundle_v2(tmp_out_dir, in_path, out_path,
                                  manifest_out_path, args, excludes)