Fix Linux omnibox handling of text with schemes

Fixes a regression in Linux handling of omnibox text like dict:word.
Regression caused omnibox to default to treating it as a url. This
CL makes Linux default to treating it as search text.

Regression introduced in https://chromium-review.googlesource.com/c/chromium/src/+/966896

Bug: 838570
Change-Id: I26be4279605ded6e3aed4e631f503e2b60551283
Reviewed-on: https://chromium-review.googlesource.com/1040881Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555829}

chrome/browser/autocomplete/chrome_autocomplete_scheme_classifier.cc

@@ -59,6 +59,12 @@ ChromeAutocompleteSchemeClassifier::GetInputTypeForScheme(
       return metrics::OmniboxInputType::QUERY;
 
     case ExternalProtocolHandler::UNKNOWN: {
+#if defined(OS_LINUX)
+      // Linux impl of GetApplicationNameForProtocol doesn't distinguish
+      // between URL schemes with handers and those without. This will
+      // make the default behaviour be search on Linux.
+      return metrics::OmniboxInputType::INVALID;
+#endif // defined(OS_LINUX)
       // If block state is unknown, check if there is an application registered
       // for the url scheme.
       GURL url(scheme + "://");