Fix for DiskCacheBackendTest.SimpleCacheOpenBadFile spurious failures

The test sometimes fails on windows, because entry file is not yet
closed when the test tries to write into it.

Review URL: https://codereview.chromium.org/783473002

Cr-Commit-Position: refs/heads/master@{#308085}

net/disk_cache/backend_unittest.cc

@@ -6,6 +6,7 @@
 #include "base/files/file_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/port.h"
+#include "base/run_loop.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
@@ -3282,6 +3283,10 @@ TEST_F(DiskCacheBackendTest, SimpleCacheOpenBadFile) {
   entry->Close();
   entry = NULL;
 
+  // The entry is being closed on the Simple Cache worker pool
+  disk_cache::SimpleBackendImpl::FlushWorkerPoolForTesting();
+  base::RunLoop().RunUntilIdle();
+
   // Write an invalid header for stream 0 and stream 1.
   base::FilePath entry_file1_path = cache_path_.AppendASCII(
       disk_cache::simple_util::GetFilenameFromKeyAndFileIndex(key, 0));