[Suggested files] Make ItemSuggests requests be BACKGROUND_REQUESTS

        files still appear in the launcher.

Tested: manually. Request format is valid, itemsuggest returns 200, and
Bug: 1146254
Change-Id: I8f28e813af1c8453a54d8e8373ccf787420ad699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521978Reviewed-by: Rachel Wong <wrong@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824729}

chrome/browser/ui/app_list/search/files/item_suggest_cache.cc

@@ -67,7 +67,8 @@ constexpr char kDriveScope[] = "https://www.googleapis.com/auth/drive.readonly";
 constexpr char kRequestBody[] = R"({
       'client_info': {
         'platform_type': 'CHROME_OS',
-        'scenario_type': 'CHROME_OS_ZSS_FILES'
+        'scenario_type': 'CHROME_OS_ZSS_FILES',
+        'request_type': 'BACKGROUND_REQUEST'
       },
       'max_suggestions': 10,
       'type_detail_fields': 'drive_item.title,justification.display_text'