Enables Google UI for handling mailto:// links by default

BUG=808113

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Idba2b2dd7777db94281a766030edfa5ed3bef801
Reviewed-on: https://chromium-review.googlesource.com/954057Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541871}

ios/chrome/browser/mailto/features.mm

@@ -9,4 +9,4 @@
 #endif
 
 const base::Feature kMailtoHandledWithGoogleUI{
-    "kMailtoHandledWithGoogleUI", base::FEATURE_DISABLED_BY_DEFAULT};
+    "kMailtoHandledWithGoogleUI", base::FEATURE_ENABLED_BY_DEFAULT};