Enable GetDisplayMedia by default in blink

See https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/eKS6bOz9a_o
for Intent to Ship and lgtms.

Bug: 326740
Change-Id: I2d8cec26bae32281e50137dfd1616926b66e61a8
Reviewed-on: https://chromium-review.googlesource.com/c/1292439
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607287}

android_webview/tools/system_webview_shell/test/data/webexposed/global-interface-listing-expected.txt

@@ -3556,6 +3556,7 @@ interface MediaDevices : EventTarget
     getter ondevicechange
     method constructor
     method enumerateDevices
+    method getDisplayMedia
     method getSupportedConstraints
     method getUserMedia
     setter ondevicechange

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3742,6 +3742,7 @@ interface MediaDevices : EventTarget
     getter ondevicechange
     method constructor
     method enumerateDevices
+    method getDisplayMedia
     method getSupportedConstraints
     method getUserMedia
     setter ondevicechange

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -543,7 +543,7 @@
     },
     {
       name: "GetDisplayMedia",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "HeapCompaction",