media_galleries: fix an bugprone-string-integer-assignment instance

details.gallery_id is a `std::string`, but `gallery_id` is an
`uint64_t`. As it stands, this assignment makes `gallery_id` equal to a
single-character string, with the character value `(char)gallery_id`.
This does not appear to be the intent.

Bug: 1130867
Change-Id: Ic8a7ce58a8a452c8890a91bdc007234ae18b73c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424815Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810146}

chrome/browser/apps/platform_apps/api/media_galleries/media_galleries_api.cc

@@ -374,7 +374,7 @@ void MediaGalleriesEventRouter::OnGalleryWatchDropped(
     MediaGalleryPrefId gallery_id) {
   MediaGalleries::GalleryChangeDetails details;
   details.type = MediaGalleries::GALLERY_CHANGE_TYPE_WATCH_DROPPED;
-  details.gallery_id = gallery_id;
+  details.gallery_id = base::NumberToString(gallery_id);
   DispatchEventToExtension(
       extension_id, extensions::events::MEDIA_GALLERIES_ON_GALLERY_CHANGED,
       MediaGalleries::OnGalleryChanged::kEventName,