A second attempt at fixing SimpleCacheCalculateSizeOfEntriesBetween

This is a second attempt to fix a unittest race.

The main thread gets a callback to update its total cache size that is
posted after the cache entry is created. Wait for that to be run
before checking the cache size.

I was able to reproduce the issue this time and can confirm that this
fixes it.

Bug: 1042441
Change-Id: Ic3f0f5a05415ba391be2887a3597c9645faac661
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015738Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Josh Karlin <jkarlin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734974}

net/disk_cache/backend_unittest.cc

@@ -2324,6 +2324,7 @@ void DiskCacheBackendTest::BackendCalculateSizeOfEntriesBetween(
   ASSERT_THAT(CreateEntry("first", &entry), IsOk());
   entry->Close();
   FlushQueueForTest();
+  base::RunLoop().RunUntilIdle();
 
   AddDelay();
   Time middle = Time::Now();
@@ -2334,6 +2335,7 @@ void DiskCacheBackendTest::BackendCalculateSizeOfEntriesBetween(
   ASSERT_THAT(CreateEntry("third_entry", &entry), IsOk());
   entry->Close();
   FlushQueueForTest();
+  base::RunLoop().RunUntilIdle();
 
   AddDelay();
   Time end = Time::Now();