Change ScrollbarGutter to experimental from test

This CL changes the status of the scrollbar-gutter CSS property
to "experimental" and cleans up related code in LayoutBlockFlow.

The Chromium feature is:
https://www.chromestatus.com/feature/5746559209701376
The intent-to-prototype thread is:
https://groups.google.com/a/chromium.org/g/blink-dev/c/GErJ9WbxJrU/m/F8N2xl1kBgAJ

Bug: 710214
Change-Id: I60fb65c91a910fb3257869fca615a6ba09340a34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462886
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815913}

third_party/blink/renderer/core/layout/layout_block_flow.cc

@@ -693,12 +693,10 @@ void LayoutBlockFlow::DetermineLogicalLeftPositionForChild(LayoutBox& child) {
   LayoutUnit total_available_logical_width =
       BorderAndPaddingLogicalWidth() + AvailableLogicalWidth();
 
-  if (StyleRef().IsHorizontalWritingMode() &&
-      !StyleRef().IsLeftToRightDirection()) {
+  if (ShouldPlaceBlockDirectionScrollbarOnLogicalLeft())
     start_position -= LogicalLeftScrollbarWidth();
-  } else {
+  else
     start_position += LogicalLeftScrollbarWidth();
-  }
 
   LayoutUnit child_margin_start = MarginStartForChild(child);
   LayoutUnit new_position = start_position + child_margin_start;

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1668,7 +1668,7 @@
     },
     {
       name: "ScrollbarGutter",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "ScrollCustomization",