Turn ScrollBoundaryBehavior runtime feature on by default

Blink intent-to-ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/OqBNF2efmFA/3ByBUKyaCgAJ
Draft Specification: https://wicg.github.io/scroll-boundary-behavior/

Bug: 672921
Change-Id: I18b1475def2346faeb60aa46085328f4d60f8470
Reviewed-on: https://chromium-review.googlesource.com/663624
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502907}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/css-properties-as-js-properties-expected.txt

@@ -241,6 +241,9 @@ right
 rx
 ry
 scrollBehavior
+scrollBoundaryBehavior
+scrollBoundaryBehaviorX
+scrollBoundaryBehaviorY
 setProperty
 shapeImageThreshold
 shapeMargin

third_party/WebKit/LayoutTests/virtual/stable/webexposed/css-property-listing-expected.txt

@@ -281,6 +281,8 @@ All changes to this list should go through Blink's feature review process: http:
     rx
     ry
     scroll-behavior
+    scroll-boundary-behavior-x
+    scroll-boundary-behavior-y
     shape-image-threshold
     shape-margin
     shape-outside
@@ -583,6 +585,9 @@ All changes to this list should go through Blink's feature review process: http:
     place-self
         align-self
         justify-self
+    scroll-boundary-behavior
+        scroll-boundary-behavior-x
+        scroll-boundary-behavior-y
     text-decoration
         text-decoration-color
         text-decoration-line

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -257,7 +257,7 @@
     },
     {
       name: "CSSScrollBoundaryBehavior",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "CSSScrollSnapPoints",