Turning CSSScrollSnapPoints experimental.

As we have implemented Snap at GestureScrollEnd on main thread,
https://chromium-review.googlesource.com/c/chromium/src/+/756887, we'd
like to turn the flag to experimental so it would be easier to test the
feature on Canary.

Bug: 497851
Change-Id: Ibdd23f17cbbbc47cbbe6d52bc492c2d910f32ce8
Reviewed-on: https://chromium-review.googlesource.com/836463Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Sandra Sun <sunyunjia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526719}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -273,7 +273,7 @@
     },
     {
       name: "CSSScrollSnapPoints",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "CSSSnapSize",