Fixed a reentrance issue in DownloadManagerImpl::PostInitialization().

DownloadManagerImpl::PostInitialization() is called twice, one by history db,
one by download db.
If History db is initialized first, it will call OnHistoryQueryComplete() and
set load_history_downloads_cb_. If download db is ready later, it calls
PostInitialization() and executes load_history_downloads_cb_ immediately.
Since load_history_downloads_cb_ calls PostInitialization() again, we got an reentrance
issue.

BUG=900417

Change-Id: Ie89146f9bd7d30ca955601c7288e0f33c2e9e11b
Reviewed-on: https://chromium-review.googlesource.com/c/1309218Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604386}

content/browser/download/download_manager_impl.cc

@@ -1012,8 +1012,11 @@ void DownloadManagerImpl::PostInitialization(
       break;
     case DOWNLOAD_INITIALIZATION_DEPENDENCY_IN_PROGRESS_CACHE:
       in_progress_cache_initialized_ = true;
-      if (load_history_downloads_cb_)
-        std::move(load_history_downloads_cb_).Run();
+      // Post a task to load downloads from history db.
+      if (load_history_downloads_cb_) {
+        base::ThreadTaskRunnerHandle::Get()->PostTask(
+            FROM_HERE, std::move(load_history_downloads_cb_));
+      }
       break;
     case DOWNLOAD_INITIALIZATION_DEPENDENCY_NONE:
     default: