Files SWA: Create browser proxy for chrome.notifications

Implements the browser proxy (a.k.a. adapter) pattern to add an
xfm.notifications API that behaves the same as chrome.notifications API
for the legacy File Manager application but suppresses generation of
system notifications when run in the SWA File manager.

Bug: b/187656842
Change-Id: I459e8893472d712fe114de93bb08b9fbef6ff3c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2947312
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Bo Majewski <majewski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#891498}

diff --git a/ui/file_manager/file_manager/BUILD.gn b/ui/file_manager/file_manager/BUILD.gn
index 1c9b8506..5811f4d 100644
--- a/ui/file_manager/file_manager/BUILD.gn
+++ b/ui/file_manager/file_manager/BUILD.gn

@@ -26,6 +26,9 @@
   input_files = [
     "main.html",
 
+    # For video player unittest:
+    "common/js/notifications_browser_proxy.js",
+
     # CSS:
     "foreground/css/combobutton.css",
     "foreground/css/common.css",