Mark CSSTouchActionPanDirections as stable.

Approved Intent to ship:

https://groups.google.com/a/chromium.org/d/msg/blink-dev/bcD93CiIJHE/2a9_ibfbDQAJ

BUG=626101

Review-Url: https://codereview.chromium.org/2323823005
Cr-Commit-Position: refs/heads/master@{#417617}

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -67,7 +67,7 @@ CSSOMSmoothScroll status=experimental
 CSSPaintAPI status=experimental, depends_on=CSSTypedOM, depends_on=Worklet
 CSSSnapSize status=experimental
 CSSStickyPosition status=experimental
-CSSTouchActionPanDirections status=experimental
+CSSTouchActionPanDirections status=stable
 CSSTypedOM status=experimental
 CSSVariables2 status=experimental
 CSSViewport status=experimental