Switch some fields in omnibox.mojom to non-nullable.

BUG=406580
TEST=None

Review URL: https://codereview.chromium.org/501143003

Cr-Commit-Position: refs/heads/master@{#291777}

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

@@ -5,8 +5,8 @@
 // The structures here roughly mirror those from autocomplete.
 
 struct AutocompleteAdditionalInfo {
-  string? key;
-  string? value;
+  string key;
+  string value;
 };
 
 struct AutocompleteMatchMojo {
@@ -15,40 +15,43 @@ struct AutocompleteMatchMojo {
   bool provider_done;
   int32 relevance;
   bool deletable;
-  string? fill_into_edit;
-  string? inline_autocompletion;
-  string? destination_url;
-  string? contents;
-  string? description;
+  string fill_into_edit;
+  string inline_autocompletion;
+  string destination_url;
+  string contents;
+  string description;
   int32 transition;
   bool is_history_what_you_typed_match;
   bool allowed_to_be_default_match;
-  string? type;
+  string type;
   string? associated_keyword;
-  string? keyword;
+  string keyword;
   bool starred;
   int32 duplicates;
   bool from_previous;
-  AutocompleteAdditionalInfo?[]? additional_info;
+  AutocompleteAdditionalInfo[] additional_info;
 };
 
 struct AutocompleteResultsForProviderMojo {
-  string? provider_name;
-  AutocompleteMatchMojo?[]? results;
+  string provider_name;
+  AutocompleteMatchMojo[] results;
 };
 
 struct OmniboxResultMojo {
   bool done;
   // Time delta since the request was started, in milliseconds.
   int32 time_since_omnibox_started_ms;
-  string? host;
+  string host;
   bool is_typed_host;
-  AutocompleteMatchMojo?[]? combined_results;
-  AutocompleteResultsForProviderMojo?[]? results_by_provider;
+  AutocompleteMatchMojo[] combined_results;
+  AutocompleteResultsForProviderMojo[] results_by_provider;
 };
 
 [Client=OmniboxPage]
 interface OmniboxUIHandlerMojo {
+  // TODO(yzshen): Conceptually |input_string| could be non-nullable. However,
+  // crbug.com/407258 (JavaScript encoder encodes empty string as null) prevents
+  // us from doing so. Change it into non-nullable once the bug is resolved.
   StartOmniboxQuery(string? input_string,
                     int32 cursor_position,
                     bool prevent_inline_autocomplete,
@@ -57,5 +60,5 @@ interface OmniboxUIHandlerMojo {
 };
 
 interface OmniboxPage {
-  HandleNewAutocompleteResult(OmniboxResultMojo? result);
+  HandleNewAutocompleteResult(OmniboxResultMojo result);
 };