Enable RangeGet_* on Fuchsia to verify if they are still flaky

Two RangeGet tests were disabled in net_unittests on Fuchsia because
they were flaky. It's likely that these two tests were fixed with
http://crrev.com/500757 (which added IO event handling in
RunLoop::RunUntilIdle()). Enable them to see if they are still flaky.

Bug: 755552
Change-Id: I96146aa7e8247aff764853de656d9997d3cf5028
Reviewed-on: https://chromium-review.googlesource.com/663798Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501500}

testing/buildbot/filters/fuchsia.net_unittests.filter

@@ -12,10 +12,6 @@
 -DiskCacheBackendTest.SimpleCacheDeleteQuickly
 -URLRequestTestHTTP.GetTest_ManyCookies
 
-# https://crbug.com/755552
--HttpCache.RangeGET_ParallelValidationDifferentRanges
--HttpCache.RangeGET_ParallelValidationOverlappingRanges
-
 # base::MakeFileUnreadable() doesn't work. https://crbug.com/759853
 -HttpNetworkTransactionTest.UploadUnreadableFile
 -SpdyNetworkTransactionTest.UnreadableFilePost