SimpleFdLimit test: serialize Close ops at the end.

If the closes run in parallel, there is some non-determinism in timing of
when they re-acquire FDs to finalize writes to disk and release FDs
afterwards, making exact behavior of FD limiter dependent on execution
schedule, especially on number of threads ScopedTaskEnvironment uses.

Change-Id: Id0da08ba64d24816835b155440cb7327dfcdf889
Reviewed-on: https://chromium-review.googlesource.com/c/1459519Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630116}

net/disk_cache/backend_unittest.cc

@@ -4653,20 +4653,21 @@ TEST_F(DiskCacheBackendTest, SimpleFdLimit) {
 
   for (int i = 0; i < kLargeNumEntries; ++i) {
     entries[i]->Close();
+    RunUntilIdle();
   }
   alt_entry->Close();
   RunUntilIdle();
 
   // Closes have to pull things in to write out the footer, but they also
-  // free up FDs, so we will only need to kick one more thing out.
+  // free up FDs.
   histogram_tester.ExpectBucketCount(
       "SimpleCache.FileDescriptorLimiterAction",
       disk_cache::FD_LIMIT_CLOSE_FILE,
-      kLargeNumEntries - 64 + 1 + kLargeNumEntries - 1 + 2 + 1);
+      kLargeNumEntries - 64 + 1 + kLargeNumEntries - 1 + 2);
   histogram_tester.ExpectBucketCount(
       "SimpleCache.FileDescriptorLimiterAction",
       disk_cache::FD_LIMIT_REOPEN_FILE,
-      kLargeNumEntries - 64 + 1 + kLargeNumEntries - 1 + 2 + 1);
+      kLargeNumEntries - 64 + 1 + kLargeNumEntries - 1 + 2);
   histogram_tester.ExpectBucketCount("SimpleCache.FileDescriptorLimiterAction",
                                      disk_cache::FD_LIMIT_FAIL_REOPEN_FILE, 0);
 }