Update field name to sync with the server side

Field name for downloading stage should be in sync with the server
side proto. It is |downloading_stage| on the server side, hence
the field name on client side should be downloadingStage.
Currently it is downloadStage.

For reference, server side proto is:
https://source.corp.google.com/piper///depot/google3/chrome/cros/reporting/api/proto/extension_install_events.proto;l=55?q=chrome%2Fcros%2Freporting%2Fapi%2Fproto%2Fextension_install_events.proto&ct=os&sq=package:piper%20file:%2F%2Fdepot%2Fgoogle3%20-file:google3%2Fexperimental

Bug: 1108257
Change-Id: I4f5826beb1ea9e54b094b7a598966f72e3e8895b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356385Reviewed-by: Anqing Zhao <anqing@chromium.org>
Commit-Queue: Swapnil Gupta <swapnilgupta@google.com>
Cr-Commit-Position: refs/heads/master@{#798731}

chrome/browser/chromeos/policy/install_event_log_util.cc

@@ -41,7 +41,7 @@ constexpr char kAndroidAppInstallEvent[] = "androidAppInstallEvent";
 // Chrome Reporting API.
 constexpr char kExtensionId[] = "extensionId";
 constexpr char kExtensionInstallEvent[] = "extensionAppInstallEvent";
-constexpr char kDownloadStage[] = "downloadStage";
+constexpr char kDownloadingStage[] = "downloadingStage";
 constexpr char kFailureReason[] = "failureReason";
 constexpr char kInstallationStage[] = "installationStage";
 constexpr char kExtensionType[] = "extensionType";
@@ -166,7 +166,7 @@ base::Value ConvertExtensionEventToValue(
   }
 
   if (extension_install_report_log_event.has_downloading_stage()) {
-    event.SetIntKey(kDownloadStage,
+    event.SetIntKey(kDownloadingStage,
                     extension_install_report_log_event.downloading_stage());
   }