Fix and re-enable simplecall-no-ssrcs.https.html.

With the changes, the test should pass regardless of if the Chrome or
Firefox behavior is used for what to do when no MSIDs are signaled.

Follow-up work is needed to test both the case of "a=msid:-" and no
"a=msid" line whatsoever, awaiting spec issue to be resolved:
https://github.com/w3c/webrtc-pc/issues/2027

// Relying on fippo's approval to land.
TBR=hta@chromium.org

Bug: 901711
Change-Id: Icaf089d206413510aadad54a0c10b2674450385a
Reviewed-on: https://chromium-review.googlesource.com/c/1319612Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605678}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3002,7 +3002,6 @@ crbug.com/893480 external/wpt/infrastructure/testdriver/actions/eventOrder.html
 crbug.com/893480 external/wpt/infrastructure/testdriver/actions/multiDevice.html [ Failure ]
 
 # ====== New tests from wpt-importer added here ======
-crbug.com/626703 [ Mac10.10 Mac10.12 Mac10.13 Retina ] virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/simplecall-no-ssrcs.https.html [ Failure Timeout ]
 crbug.com/626703 external/wpt/css/css-text/letter-spacing/letter-spacing-bidi-002.html [ Failure ]
 crbug.com/626703 external/wpt/css/css-text/letter-spacing/letter-spacing-nesting-002.html [ Failure ]
 crbug.com/626703 [ Win ] external/wpt/css/css-text/word-break/word-break-keep-all-005.html [ Failure ]

third_party/WebKit/LayoutTests/external/wpt/webrtc/simplecall-no-ssrcs.https.html

@@ -101,9 +101,9 @@
 
   var onRemoteTrack = test.step_func(function(event) {
     var videoTag = document.getElementById('remote-view');
-    if (!videoTag.srcObject) {
-      videoTag.srcObject = event.streams[0];
-    }
+    if (!videoTag.srcObject)
+      videoTag.srcObject = new MediaStream();
+    videoTag.srcObject.addTrack(event.track);
   });
 
   // Returns a suitable error callback.

third_party/WebKit/LayoutTests/platform/mac-mac10.12/virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/simplecall-no-ssrcs.https-expected.txt

-This is a testharness.js-based test.
-TIMEOUT Can set up a basic WebRTC call without announcing ssrcs. Test timed out
-Harness: the test ran to completion.
-

third_party/WebKit/LayoutTests/platform/win/virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/simplecall-no-ssrcs.https-expected.txt

-This is a testharness.js-based test.
-TIMEOUT Can set up a basic WebRTC call without announcing ssrcs. Test timed out
-Harness: the test ran to completion.
-