Files app: Force create_test_main.py to run as python2

This script is failing with Python 3.

Bug: 1163386
Change-Id: I7d785a1b13ef7fbb91c203be768ba682cc3d6f85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612625
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840492}

ui/file_manager/file_manager/test/BUILD.gn

@@ -4,7 +4,7 @@
 
 import("//third_party/closure_compiler/compile_js.gni")
 
-action("create_test_main") {
+python2_action("create_test_main") {
   script = "//ui/file_manager/file_manager/test/scripts/create_test_main.py"
   output = "$target_gen_dir/../test.html"
   sources = [