Add 3p origin trials config for conversion measurement

What: Add relevant config line for 3p origin trials

Why: The Conversion Measurement API may be used by third parties to
measure conversions between different first parties. Allowing them to
opt-in makes the trial integration much easier.

Bug: 1113879
Change-Id: I76df331041d0a3519c0805c7d8a9239556fcd492
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340553Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796019}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -358,6 +358,7 @@
     {
       name: "ConversionMeasurement",
       origin_trial_feature_name: "ConversionMeasurement",
+      origin_trial_allows_third_party: true,
       status: "experimental",
     },
     {