Plug file leaks in GCMDriverTest.

Unregistering ends up posting tasks to perform cleanup in the Encryption
db. Let these tasks run during TearDown so that all files have been
closed by the time the test fixture's ScopedTempDir is torn down.

BUG=546640

Change-Id: Iaea6142c74cf4db65809403ecc681ebd1243ddb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942427
Auto-Submit: Greg Thompson <grt@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719968}

components/gcm_driver/gcm_driver_desktop_unittest.cc

@@ -212,6 +212,7 @@ void GCMDriverTest::TearDown() {
   PumpIOLoop();
 
   io_thread_.Stop();
+  task_environment_.RunUntilIdle();
 }
 
 void GCMDriverTest::PumpIOLoop() {