Add WebCodecs OT flag

This CL allows the WebCodecs feature flag to be controlled by origin
trial infrastructure. It also bumps up WebCodecs to be part of the
experimental Web Platform features.

Bug: 1120704
Change-Id: I1ce31847ba467bdfd00e5ff2c6f4289c6d69fe53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368232
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800732}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2035,7 +2035,8 @@
     },
     {
       name: "WebCodecs",
-      status: "test",
+      status: "experimental",
+      origin_trial_feature_name: "WebCodecs"
     },
     {
       name: "WebCryptoCurve25519",