media-app: Add missing 48px icon as a resource.

The WebAppProvider system couldn't download it given the url from the manifest:
chrome://media-app/system_assets/app_icon_48.png

It would be nice to have some offline integrity checks for SWAs metadata.

Fixed: 1110768
Change-Id: Ie8fb1b2bc06bb9f64fdc2d0c22bafc3cb43da4f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367589Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Auto-Submit: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800518}

chromeos/components/media_app_ui/media_app_ui.cc

@@ -50,6 +50,8 @@ content::WebUIDataSource* CreateHostDataSource() {
                           IDR_MEDIA_APP_GALLERY_ICON_16_PNG);
   source->AddResourcePath("system_assets/app_icon_32.png",
                           IDR_MEDIA_APP_GALLERY_ICON_32_PNG);
+  source->AddResourcePath("system_assets/app_icon_48.png",
+                          IDR_MEDIA_APP_GALLERY_ICON_48_PNG);
   source->AddResourcePath("system_assets/app_icon_64.png",
                           IDR_MEDIA_APP_GALLERY_ICON_64_PNG);
   source->AddResourcePath("system_assets/app_icon_96.png",