component updater: add temporary VLOG to CUS

I suspect throttling is preventing CUS from updating crostini. Add this
temporary VLOG to confirm this.

BUG=chromium:851151
TEST=None

Change-Id: I505214071b9f39c4ab3f95e96d5183667901d853
Reviewed-on: https://chromium-review.googlesource.com/1123261
Commit-Queue: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572036}

components/update_client/update_engine.cc

@@ -82,6 +82,11 @@ void UpdateEngine::Update(bool is_foreground,
   }
 
   if (IsThrottled(is_foreground)) {
+    // TODO(xiaochu): remove this log after https://crbug.com/851151 is fixed.
+    VLOG(1) << "Background update is throttled for following components:";
+    for (const auto& id : ids) {
+      VLOG(1) << "id:" << id;
+    }
     base::ThreadTaskRunnerHandle::Get()->PostTask(
         FROM_HERE, base::BindOnce(std::move(callback), Error::RETRY_LATER));
     return;