Add idle-detection mojom.js files to mojojs.zip

These files need to be accessible when running wpt
outside chromium environment.

see: https://chromium.googlesource.com/chromium/src/+/master/docs/testing/web_platform_tests.md#mojojs

This is the first step towards making these tests pass outside the Chromium CI

Bug: 1123990
Change-Id: Id96a3a2ae01beb6d9fd37eb41acad1ab0ebacf76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2396219
Commit-Queue: Arnaud Mandy <arnaud.mandy@intel.com>
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805069}

chrome/tools/build/linux/FILES.cfg

@@ -328,6 +328,12 @@ FILES = [
     'buildtype': ['dev', 'official'],
     'archive': 'mojojs.zip',
   },
+  # MojoJS support data (idle-detection):
+  {
+    'filename': 'gen/third_party/blink/public/mojom/idle/idle_manager.mojom.js',
+    'buildtype': ['dev', 'official'],
+    'archive': 'mojojs.zip',
+  },
   # MojoJS support data (Image Capture)
   {
     'filename': 'gen/media/capture/mojom/image_capture.mojom.js',