Files Next: Enable resource remapping for Jelly

Changes the webUI resource loading code to look for file names with a
'_gm3' at the end of their basename and sets the loading id map to point
to them instead of the original resource name (gated behind Jelly flag).

Bug: b/242932747
Change-Id: I9e7dae71a757e4e42052b2c874609b703805aae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3868434
Reviewed-by: Wenbo Jie <wenbojie@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1042448}

diff --git a/ui/file_manager/file_manager/BUILD.gn b/ui/file_manager/file_manager/BUILD.gn
index 3278816..764af65 100644
--- a/ui/file_manager/file_manager/BUILD.gn
+++ b/ui/file_manager/file_manager/BUILD.gn

@@ -47,6 +47,7 @@
     "foreground/css/combobutton.css",
     "foreground/css/common.css",
     "foreground/css/file_manager.css",
+    "foreground/css/file_manager_gm3.css",
     "foreground/css/file_status.css",
     "foreground/css/file_types.css",
     "foreground/css/list.css",