nostate-prefetch: Enable by default

This would allow M66 cap in the Finch config. Removing the Feature and
OmniboxMode/etc will be slightly easier to do when Prerender gets
hard-disabled, postponing the cleanup until then.

Bug: 762489
Change-Id: Ica411ec05a211c8aa3bf779b6c8764403a36308d
Reviewed-on: https://chromium-review.googlesource.com/895764Reviewed-by: Matthew Cary <mattcary@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533299}

chrome/browser/prerender/prerender_field_trial.cc

@@ -68,9 +68,9 @@ const base::Feature kNoStatePrefetchFeature{"NoStatePrefetch",
                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 void ConfigurePrerender() {
-  PrerenderManager::PrerenderManagerMode overall_mode = ParsePrerenderMode(
-      kNoStatePrefetchFeatureModeParameterName,
-      PrerenderManager::PRERENDER_MODE_SIMPLE_LOAD_EXPERIMENT);
+  PrerenderManager::PrerenderManagerMode overall_mode =
+      ParsePrerenderMode(kNoStatePrefetchFeatureModeParameterName,
+                         PrerenderManager::PRERENDER_MODE_NOSTATE_PREFETCH);
 
   PrerenderManager::SetMode(overall_mode);
   PrerenderManager::SetOmniboxMode(ParsePrerenderMode(