Flush the message loop in StorageMonitorLinux tests.

chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc

Index: chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc
diff --git a/chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc b/chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc
index 1a1991862b0d29989e1860ecd994efabbb03ee37..7a74069defa2d3504e9b40059e96e00a44ad57a5 100644
--- a/chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc
+++ b/chrome/browser/storage_monitor/storage_monitor_linux_unittest.cc
@@ -204,6 +204,7 @@ class StorageMonitorLinuxTest : public testing::Test {
 
     // Linux storage monitor must be destroyed on the UI thread, so do it here.
     test::TestStorageMonitor::RemoveSingleton();
+    base::RunLoop().RunUntilIdle();
   }
 
   // Append mtab entries from the |data| array of size |data_size| to the mtab