[Imperative Slot API] Ship Imperative Slot API

Intent to ship:
https://groups.google.com/a/chromium.org/g/blink-dev/c/Jdw3VWbKvLY/m/0YBXYr-lBwAJ

Bug: 869308
Change-Id: I75cab8d0c7c117ce67a8962a903b534e11620f95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358608Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799457}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1047,7 +1047,7 @@
     },
     {
       name:"ManualSlotting",
-      status:"experimental",
+      status:"stable",
     },
     {
       name: "MathMLCore",

third_party/blink/web_tests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -923,6 +923,7 @@ html element select
 html element shadow
     property getDistributedNodes
 html element slot
+    property assign
     property assignedElements
     property assignedNodes
     property name

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3448,6 +3448,7 @@ interface HTMLShadowElement : HTMLElement
 interface HTMLSlotElement : HTMLElement
     attribute @@toStringTag
     getter name
+    method assign
     method assignedElements
     method assignedNodes
     method constructor
@@ -6938,6 +6939,7 @@ interface ShadowRoot : DocumentFragment
     getter mode
     getter pictureInPictureElement
     getter pointerLockElement
+    getter slotAssignment
     getter styleSheets
     method constructor
     method elementFromPoint