Revert "Disable flaky test ArcFileSystemBridgeTest.GetFileName"

This reverts commit 8f6536d9.

Reason for revert: root caused fixed by crrev.com/644548
As mentioend @ https://bugs.chromium.org/p/chromium/issues/detail?id=938126#c42

Original change's description:
> Disable flaky test ArcFileSystemBridgeTest.GetFileName
> 
> Test has been identified as flaky by Findit. Analysis can be found
> in the referenced bug. Disabling the test as too much time passed
> since first flake occurred and finding the CL to revert is going to
> be to long.
> 
> TBR=nya@chromium.org
> 
> Bug: 939935
> To-Be-Reviewed: nya@chromium.org
> Change-Id: Ide6ac6b1333a71034f49779e659c32e70a1fe298
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1514678
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#639478}

TBR=sdefresne@chromium.org,nya@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 939935
Change-Id: I0aa031e2e259c324f21dc6687dac7808815e0693
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542037Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644878}

chrome/browser/chromeos/arc/fileapi/arc_file_system_bridge_unittest.cc

@@ -92,8 +92,7 @@ class ArcFileSystemBridgeTest : public testing::Test {
   std::unique_ptr<ArcFileSystemBridge> arc_file_system_bridge_;
 };
 
-// TODO(crbug.com/939935): test is flaky, disabled.
-TEST_F(ArcFileSystemBridgeTest, DISABLED_GetFileName) {
+TEST_F(ArcFileSystemBridgeTest, GetFileName) {
   base::RunLoop run_loop;
   arc_file_system_bridge_->GetFileName(
       EncodeToChromeContentProviderUrl(GURL(kTestUrl)).spec(),