Implement PlatformUtil::OpenExternal() so that mailto: can work

chrome/chrome_browser.gypi

 # Copyright 2012 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.
 
 {
   'variables': {
     # All .cc, .h, .m, and .mm files under browser except for:
     # * tests and mocks.
     # * code below browser/chromeos
     # * code below browser/extensions
@@ skipping 1759 matching lines @@
       'android/java/src/org/chromium/chrome/browser/infobar/DownloadOverwriteInfoBar.java',
       'android/java/src/org/chromium/chrome/browser/infobar/TranslateInfoBar.java',
       'android/java/src/org/chromium/chrome/browser/net/qualityprovider/ExternalEstimateProviderAndroid.java',
       'android/java/src/org/chromium/chrome/browser/notifications/NotificationUIManager.java',
       'android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageBridge.java',
       'android/java/src/org/chromium/chrome/browser/omnibox/AnswersImage.java',
       'android/java/src/org/chromium/chrome/browser/omnibox/AutocompleteController.java',
       'android/java/src/org/chromium/chrome/browser/omnibox/OmniboxPrerender.java',
       'android/java/src/org/chromium/chrome/browser/omnibox/OmniboxUrlEmphasizer.java',
       'android/java/src/org/chromium/chrome/browser/omnibox/OmniboxViewUtil.java',
+      'android/java/src/org/chromium/chrome/browser/PlatformUtil.java',
       'android/java/src/org/chromium/chrome/browser/partnerbookmarks/PartnerBookmarksReader.java',
       'android/java/src/org/chromium/chrome/browser/password_manager/Credential.java',
       'android/java/src/org/chromium/chrome/browser/PasswordUIView.java',
       'android/java/src/org/chromium/chrome/browser/precache/PrecacheLauncher.java',
       'android/java/src/org/chromium/chrome/browser/preferences/autofill/AutofillProfileBridge.java',
       'android/java/src/org/chromium/chrome/browser/preferences/LocationSettings.java',
       'android/java/src/org/chromium/chrome/browser/preferences/PrefServiceBridge.java',
       'android/java/src/org/chromium/chrome/browser/preferences/website/WebsitePreferenceBridge.java',
       'android/java/src/org/chromium/chrome/browser/profiles/MostVisitedSites.java',
       'android/java/src/org/chromium/chrome/browser/profiles/Profile.java',
@@ skipping 2327 matching lines @@
             '../components/components.gyp:omnibox_browser',
             '../components/components.gyp:rlz',
             '../components/components.gyp:search_engines',
             '../rlz/rlz.gyp:rlz_lib',
           ],
         },
       ],
     }],
   ],
 }