Fix a scoped temp dir leak in ImportantSitesUsageCounterTest

The SQL DB owned by the QuotaManager gets destroyed asynchronously, so
given the current ordering of the parameters:
  base::ScopedTempDir temp_dir_;
  scoped_refptr<QuotaManager> quota_manager_;

The release of |quota_manager_| will post a task to close the database
stored in temp_dir_ but this is racy with the destruction of
|temp_dir_|.

Bug: 974310
Change-Id: Ib54ea1fe1652028a1bcf6b7f6107a01d3e99f2d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660833
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672215}

chrome/browser/engagement/important_sites_usage_counter_unittest.cc

@@ -34,7 +34,11 @@ class ImportantSitesUsageCounterTest : public testing::Test {
     run_loop_.reset(new base::RunLoop());
   }
 
-  void TearDown() override { content::RunAllTasksUntilIdle(); }
+  void TearDown() override {
+    // Release the quota manager and wait for the database to be closed.
+    quota_manager_.reset();
+    content::RunAllTasksUntilIdle();
+  }
 
   TestingProfile* profile() { return &profile_; }