1. Add requestFileSystemAccessSync(), which will replace allowFileSystem() in the future. 
2. Rename requestFileSystemAccess to requestFileSystemAccessAsync.

This is the first patch for user story of "file system permission in webview implementation".

BUG=343382

Review URL: https://codereview.chromium.org/289793002

git-svn-id: svn://svn.chromium.org/blink/trunk@174223 bbb929c8-8fbe-4397-9dbb-9b2b20218538