Enable OmniboxSpeculativeServiceWorkerStartOnQueryInput by default on all platforms.

This has been enabled on Android since Chrome 62 by server-side field
trial configuation, and since Chrome 64 by the client code. As part of
the launch process for issue 733466, we received approval to enable it
on all platforms after Android 62 rolled out widely, which is definitely
true now.

This CL still keeps the feature flag, but it should be removed completely later.

Bug: 733466, 819476
Change-Id: Idcd85b1afb3e7724a1ff6a3b9cd8e1db6ec23788
Reviewed-on: https://chromium-review.googlesource.com/952955Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541744}

components/omnibox/browser/omnibox_field_trial.cc

@@ -144,11 +144,7 @@ const base::Feature kUIExperimentVerticalMargin{
 // query.
 const base::Feature kSpeculativeServiceWorkerStartOnQueryInput{
   "OmniboxSpeculativeServiceWorkerStartOnQueryInput",
-#if defined(OS_ANDROID)
       base::FEATURE_ENABLED_BY_DEFAULT
-#else
-      base::FEATURE_DISABLED_BY_DEFAULT
-#endif
 };
 
 // Feature used to allow breaking words at underscores in building