Enable MediaDevices.enumerateDevices and let promises-based getUserMedia behind a flag.

Source/platform/RuntimeEnabledFeatures.in

Index: Source/platform/RuntimeEnabledFeatures.in
diff --git a/Source/platform/RuntimeEnabledFeatures.in b/Source/platform/RuntimeEnabledFeatures.in
index 176a7186e13521a1bf4bb91f2d8513da25fae827..3c22e2d4df6bad986d56c31dfa34976175c5722e 100644
--- a/Source/platform/RuntimeEnabledFeatures.in
+++ b/Source/platform/RuntimeEnabledFeatures.in
@@ -63,7 +63,6 @@ DisplayList2dCanvas status=stable
 DurableStorage status=experimental
 ForceDisplayList2dCanvas
 EncryptedMedia status=stable
-EnumerateDevices status=experimental
 ExecCommandInJavaScript status=test
 ComputedAccessibilityInfo status=experimental
 ExperimentalCanvasFeatures status=test
@@ -76,6 +75,7 @@ FullscreenUnprefixed status=test
 FrameTimingSupport status=experimental
 Geofencing status=experimental
 GeometryInterfaces status=test
+GetUserMedia depends_on=MediaDevices, status=experimental
 GlobalCacheStorage status=stable
 ImageColorProfiles
 ImageOrientation status=test
@@ -93,6 +93,7 @@ FractionalScrollOffsets status=experimental
 Media status=stable
 MediaCapture
 MediaController depends_on=Media, status=experimental
+MediaDevices status=stable
 MediaSession
 MediaSource status=stable
 MediaSourceExperimental depends_on=MediaSource, status=experimental