Update MarkHttpAs flag field trial name

The flag still had the old "HTTPBadPhase3" trial name, which was causing
a DCHECK failure due to the conflict between the old field trial name
and the new "HTTPReallyBadFinal" field trial name.

Change-Id: I563ec74b1005aa360168d3ee918fdd3be5252d56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1500024Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638017}

chrome/browser/about_flags.cc

@@ -3182,7 +3182,7 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_WITH_PARAMS_VALUE_TYPE(
          security_state::features::kMarkHttpAsFeature,
          kMarkHttpAsFeatureVariations,
-         "HTTPBadPhase3")},
+         "HTTPReallyBadFinal")},
 
 #if !defined(OS_ANDROID)
     {"enable-web-authentication-testing-api",