Bug 1537986 - Make deviceId constraints in MST-getSettings.https.html use `exact`. r=jib

Depends on D33470

Differential Revision: https://phabricator.services.mozilla.com/D33472

--HG--
extra : moz-landing-system : lando

testing/web-platform/tests/mediacapture-streams/MediaStreamTrack-getSettings.https.html

@@ -28,7 +28,7 @@
 
     const mediaStream2 = await navigator.mediaDevices.getUserMedia({
       video: {
-        deviceId: settings1.deviceId,
+        deviceId: {exact: settings1.deviceId},
       },
       audio: false
     });
@@ -48,7 +48,7 @@
 
     const mediaStream2 = await navigator.mediaDevices.getUserMedia({
       video: {
-        deviceId: settings1.deviceId,
+        deviceId: {exact: settings1.deviceId},
         width: {
           ideal: settings1.width / 2,
         },