Enable User Activation API by default

Approved Intent to Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/vTnSjFcPwNc/kkTX92hpBgAJ

BUG=846858
TBR=timvolodine@chromium.org

Change-Id: I48c81a799130b7d89157ba0795985fc2eb88ff17
Reviewed-on: https://chromium-review.googlesource.com/c/1291996
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604938}

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

@@ -3741,6 +3741,7 @@ interface MessageEvent : Event
     getter origin
     getter ports
     getter source
+    getter userActivation
     method constructor
     method initMessageEvent
 interface MessagePort : EventTarget
@@ -3860,6 +3861,7 @@ interface Navigator
     getter product
     getter productSub
     getter serviceWorker
+    getter userActivation
     getter userAgent
     getter vendor
     getter vendorSub
@@ -5818,6 +5820,11 @@ interface URLSearchParams
     method sort
     method toString
     method values
+interface UserActivation
+    attribute @@toStringTag
+    getter hasBeenActive
+    getter isActive
+    method constructor
 interface VTTCue : TextTrackCue
     attribute @@toStringTag
     getter align

third_party/WebKit/LayoutTests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt

@@ -632,6 +632,7 @@ interface MessageEvent : Event
     getter origin
     getter ports
     getter source
+    getter userActivation
     method constructor
     method initMessageEvent
 interface MessagePort : EventTarget

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

@@ -600,6 +600,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter origin
 [Worker]     getter ports
 [Worker]     getter source
+[Worker]     getter userActivation
 [Worker]     method constructor
 [Worker]     method initMessageEvent
 [Worker] interface MessagePort : EventTarget

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

@@ -3954,6 +3954,7 @@ interface MessageEvent : Event
     getter origin
     getter ports
     getter source
+    getter userActivation
     method constructor
     method initMessageEvent
 interface MessagePort : EventTarget
@@ -4088,6 +4089,7 @@ interface Navigator
     getter serviceWorker
     getter storage
     getter usb
+    getter userActivation
     getter userAgent
     getter vendor
     getter vendorSub
@@ -6804,6 +6806,11 @@ interface USBOutTransferResult
     getter bytesWritten
     getter status
     method constructor
+interface UserActivation
+    attribute @@toStringTag
+    getter hasBeenActive
+    getter isActive
+    method constructor
 interface VTTCue : TextTrackCue
     attribute @@toStringTag
     getter align

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

@@ -595,6 +595,7 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     getter origin
 [Worker]     getter ports
 [Worker]     getter source
+[Worker]     getter userActivation
 [Worker]     method constructor
 [Worker]     method initMessageEvent
 [Worker] interface MessagePort : EventTarget

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1268,7 +1268,7 @@
     },
     {
       name: "UserActivationAPI",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "UserActivationV2",