Untie Wasm SIMD and Threads from experimental web platform features

We have:

1. kExperimentalWebAssemblyFeatures, and
2. kEnableExperimentalWebPlatformFeatures

Currently, both of them will turn on Wasm SIMD and Threads.
This is slightly confusing. Change it so that only 1 will turn on these
Wasm features. (Turning on the feature specific flag will continue to
turn on these flags.)

Bug: chromium:1149796
Change-Id: I46c9a0daf248b07333a67fe3816bb3f1981b28de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2583583Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836014}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2121,12 +2121,12 @@
     {
       name: "WebAssemblySimd",
       origin_trial_feature_name: "WebAssemblySimd",
-      status: "experimental",
+      status: "test",
     },
     {
       name: "WebAssemblyThreads",
       origin_trial_feature_name: "WebAssemblyThreads",
-      status: "experimental",
+      status: "test",
     },
     // WebAuth is disabled on Android versions prior to N (7.0) due to lack of
     // supporting APIs, see runtime_features.cc.