WebUI: Migrate file manager to preprocess_if_expr

Migrate file manager from preprocess_grit (being removed) to
preprocess_if_expr. Both tools copy files/generate manifests for
bundling and generating grd files, and both process <if expr>, but
preprocess_if_expr no longer supports the deprecated // <include
in JS.

file_manager uses <include> in JS, but not in any files being processed
by preprocess_grit, so it can be migrated to preprocess_if_expr.

Also correcting a comment about inlining images, as neither the
old preprocess_grit nor the new preprocess_if_expr inline images.

Change-Id: Ie2d1f669346183263250abf0613450c573d28943
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570455Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833498}

ui/file_manager/BUILD.gn

@@ -5,7 +5,7 @@
 import("//chrome/common/features.gni")
 import("//third_party/closure_compiler/compile_js.gni")
 import("//tools/grit/grit_rule.gni")
-import("//tools/grit/preprocess_grit.gni")
+import("//tools/grit/preprocess_if_expr.gni")
 import("//ui/webui/resources/tools/generate_grd.gni")
 
 grit("resources") {
@@ -80,10 +80,10 @@ group("unit_test_data") {
 
 preprocess_folder = "$target_gen_dir/preprocessed"
 
-# Inline images and move files to the preprocessed folder to combine all
-# resources in the same folder to be able to generate the JS bundle.
+# Move files to the preprocessed folder to combine all resources in the same
+# folder for JS bundling with rollup.
 # This is for files direct from the src/:  input_folder="./".
-preprocess_grit("preprocess_static") {
+preprocess_if_expr("preprocess_static") {
   in_folder = "./"
   out_folder = preprocess_folder
 
@@ -96,7 +96,7 @@ preprocess_grit("preprocess_static") {
 }
 
 # This is for files that are generated in "gen/": input_folder=target_gen_dir.
-preprocess_grit("preprocess_generated") {
+preprocess_if_expr("preprocess_generated") {
   in_folder = target_gen_dir
   out_folder = preprocess_folder