[vr] Bail on unhandled code points.

chrome/browser/android/vr_shell/font_fallback.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_ANDROID_VR_SHELL_FONT_FALLBACK_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_FONT_FALLBACK_H_
 
 #include <string>
 
 #include "third_party/icu/source/common/unicode/uchar.h"
 
 namespace gfx {
 class Font;
 }  // namespace gfx
 
 namespace vr_shell {
 
 // Return a font name which provides a glyph for the Unicode code point
 // specified by character.
 //   default_font: The main font for which fallbacks are required
 //   c: a UTF-32 code point
 //   preferred_locale: preferred locale identifier (if any) for |c|
 //                     (e.g. "en", "ja", "zh-CN")
 //
-// Returns: the font name. The value is empty if the request could not be
-// satisfied or if the provided default font supports it.
-std::string GetFallbackFontNameForChar(const gfx::Font& default_font,
-                                       UChar32 c,
-                                       const std::string& preferred_locale);
+// The funtion, if it succeeds, sets |font_name|. Even if it succeeds, it may
+// set |font_name| to the empty string if the character is supported by the
+// default font.
+//
+// Returns:
+//   * false, if the request could not be satisfied or if the provided default
+//     font supports it.
+//   * true, otherwis.
+//
+bool GetFallbackFontNameForChar(const gfx::Font& default_font,
+                                UChar32 c,
+                                const std::string& preferred_locale,
+                                std::string* font_name);
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_FONT_FALLBACK_H_