Change status of UserAgentClientHint to experimental

This is consistent with the chromium feature which is disabled by
default and is enabled when "experimental web platform feature" is
enabled [1].

This prepares for letting blink feature status override chrome's
default feature status.

[1] https://source.chromium.org/chromium/chromium/src/+/master:content/public/common/content_switch_dependent_feature_overrides.cc;drc=02fc05043bd38d318ba9ce59ccc4c7602284f990;l=56

Bug: 1121839
Change-Id: I92b7c68ce409971ef920ce00cb0951600547ef57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382486Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803166}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1939,7 +1939,7 @@
     },
     {
       name: "UserAgentClientHint",
-      status: "stable"
+      status: "experimental",
     },
     {
       name: "V8IdleTasks",