[Fuchsia] Un-filter some tests that we expect should no longer be flaky.

Un-filter three tests which were flaking ~1 year ago:
- DiskCacheBackendTest.InvalidEntry5
- DiskCacheEntryTest.ExternalAsyncIONoBuffer
- URLRequestTest.NetworkDelegateProxyError

There have been substantial filesystem and network changes in the
platform, and we have enabled TestLauncher retries on the Fuchsia bots,
so these should be safe to start running.

Bug: 784448, 810521, 814811
Change-Id: I7166ab60ee8a5a8dc7097c0969da8c7bc57878b8
Reviewed-on: https://chromium-review.googlesource.com/c/1405595Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622069}

testing/buildbot/filters/fuchsia.net_unittests.filter

@@ -11,22 +11,13 @@
 -SupportedVersion/QuicConnectionTest.TooManySentPackets/*
 -RangeOfTreeSizes/CTLogVerifierTestUsingGenerator.VerifiesValidConsistencyProof/*
 
-# Flaky, https://crbug.com/784448.
--DiskCacheBackendTest.InvalidEntry5
-
 # Flaky, https://crbug.com/793176.
 -NetworkQualityEstimatorTest.RecordAccuracy
 
-# Flaky, https://crbug.com/810521.
--DiskCacheEntryTest.ExternalAsyncIONoBuffer
-
 # Flaky, https://crbug.com/813631.
 -URLRequestQuicTest.CancelPushIfCached_*
 -URLRequestQuicTest.DoNotCancelPushIfNotFoundInCache
 
-# Flaky, https://crbug.com/814811.
--URLRequestTest.NetworkDelegateProxyError
-
 # Flaky hangs, https://crbug.com/913037.
 -URLRequestTestHTTP.Redirect30*