Change chrome://flags field trial name for HTTP-Bad

Make the chrome://flags field trial name match the field trial name in
fieldtrial_testing_config.json. Otherwise, if you've previously set the flag and
launch Chrome now that the field trial config is set, you hit the following
DCHECK:

Check failed: !ContainsKey(overrides_, feature_name) ||
!overrides_.find(feature_name)->second.field_trial. Feature MarkHttpAs has
conflicting field trial overrides: MarkHttpAs / HTTPBadPhase3

Change-Id: I463641c5442b15aadc8a44673396575e49cddee4
Reviewed-on: https://chromium-review.googlesource.com/901957Reviewed-by: Eric Lawrence <elawrence@chromium.org>
Commit-Queue: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534451}

chrome/browser/about_flags.cc

@@ -3692,7 +3692,7 @@ const FeatureEntry kFeatureEntries[] = {
      FEATURE_WITH_PARAMS_VALUE_TYPE(
          security_state::features::kMarkHttpAsFeature,
          kMarkHttpAsFeatureVariations,
-         "MarkHttpAs")},
+         "HTTPBadPhase3")},
 
 #if !defined(OS_ANDROID)
     {"enable-web-authentication-api",