chromeos: Update FileFlusherTest to not use BrowserThread.

Remove calls to
content::BrowserThread::GetBlockingPool()->FlushForTesting()
and base::RunLoop().RunUntilIdle() from TearDown().
content::TestBrowserThreadBundle is documented as already
running remaining tasks during destruction.

BUG=667892

Change-Id: I0368a0f94e9ddbfde284b18e3222894ee1d80b55
Reviewed-on: https://chromium-review.googlesource.com/560643Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Satoru Takabayashi <satorux@google.com>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485435}

chrome/browser/chromeos/base/file_flusher_unittest.cc

@@ -14,7 +14,6 @@
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/sequenced_worker_pool.h"
-#include "content/public/browser/browser_thread.h"
 #include "content/public/test/test_browser_thread_bundle.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -57,11 +56,6 @@ class FileFlusherTest : public testing::Test {
     }
   }
 
-  void TearDown() override {
-    content::BrowserThread::GetBlockingPool()->FlushForTesting();
-    base::RunLoop().RunUntilIdle();
-  }
-
   std::unique_ptr<FileFlusher> CreateFileFlusher() {
     std::unique_ptr<FileFlusher> flusher(new FileFlusher);
     flusher->set_on_flush_callback_for_test(