[chrome:omnibox] Display string representation for transition column enum.

Previously, the transition column displays the enum's integer representation.
The 'generated' and 'typed' transition values are most common, adjusting the
transition column width to 70px (from 60px) allows these values to be displayed
without eliding.

Bug: 891303
Change-Id: Ic356e9f89541e22fa1cdfd54959a444d0cab6836
Reviewed-on: https://chromium-review.googlesource.com/c/1374489Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616092}

chrome/browser/resources/omnibox/omnibox_column_widths.css

@@ -23,7 +23,7 @@
 }
 
 .header-transition {
-  width: 60px;
+  width: 70px;
 }
 
 .header-associated-keyword {

chrome/browser/ui/webui/omnibox/omnibox.mojom

@@ -22,7 +22,7 @@ struct AutocompleteMatch {
   string destination_url;
   string contents;
   string description;
-  int32 transition;
+  string transition;
   bool allowed_to_be_default_match;
   string type;
   bool has_tab_match;

chrome/browser/ui/webui/omnibox/omnibox_page_handler.cc

@@ -77,7 +77,8 @@ struct TypeConverter<mojom::AutocompleteMatchPtr, AutocompleteMatch> {
     // At this time, we're not bothering to send along the long vector that
     // represents description classification.  i.e., for each character, what
     // type of text it is.
-    result->transition = input.transition;
+    result->transition =
+        ui::PageTransitionGetCoreTransitionString(input.transition);
     result->allowed_to_be_default_match = input.allowed_to_be_default_match;
     result->type = AutocompleteMatchType::ToString(input.type);
     result->has_tab_match = input.has_tab_match;