WPT webrtc/getstats test: Allow 0 and 1 datachannels

There seems to be a race on whether the count of datachannels
is incremented before a session goes to "connected". This CL
allows both 0 and 1.

The test would fail ~1/4000 times on a Linux workstation.

Bug: 771540
Change-Id: Ic250443c2addef995dbb26e7cd021a516dcd023b
Reviewed-on: https://chromium-review.googlesource.com/721379Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509564}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3740,8 +3740,6 @@ crbug.com/771492 [ Win Linux Mac ] external/wpt/css/css-tables-3/table-model-fix
 # Suspended WebRTC tests
 # Test fails, and output changes for each run.
 crbug.com/771540 external/wpt/webrtc/RTCPeerConnection-setLocalDescription-answer.html [ Failure ]
-# Test is flaky after removing mock PeerConnectionHandler
-crbug.com/771540 external/wpt/webrtc/getstats.html [ Pass Failure ]
 
 # Sheriff failures 2017-10-04
 crbug.com/770691 [ Win7 Linux Mac10.11 ] media/controls/lazy-loaded-style.html [ Failure Pass ]

third_party/WebKit/LayoutTests/external/wpt/webrtc/getstats.html

@@ -68,11 +68,14 @@ This test uses data only, and thus does not require fake media devices.
       }
       showStats.innerHTML = JSON.stringify(reportDictionary, null, 2);
       // Check the stats properties.
-      assert_not_equals(report, null);
+      assert_not_equals(report, null, 'No report');
       let sessionStat = getStatsRecordByType(report, 'peer-connection');
       assert_not_equals(sessionStat, null, 'Did not find peer-connection stats');
-      assert_exists(sessionStat, 'dataChannelsOpened');
-      assert_equals(sessionStat.dataChannelsOpened, 1);
+      assert_exists(sessionStat, 'dataChannelsOpened', 'no dataChannelsOpened stat');
+      // Once every 4000 or so tests, the datachannel won't be opened when the getStats
+      // function is done, so allow both 0 and 1 datachannels.
+      assert_true(sessionStat.dataChannelsOpened == 1 || sessionStat.dataChannelsOpened == 0,
+                  'dataChannelsOpened count wrong');
       test.done();
     })
     .catch(test.step_func(function(e) {
@@ -93,7 +96,6 @@ This test uses data only, and thus does not require fake media devices.
     // The createDataChannel is necessary and sufficient to make
     // sure the ICE connection be attempted.
     gFirstConnection.createDataChannel('channel');
-
     var atStep = 'Create offer';
 
     gFirstConnection.createOffer()