FileAPI: Add interface for WebFileSystem::resolveURL

This is a preparation patch to support resolveLocalFileSystemURL on
SyncFileSystem.  And also a cleanup patch to add a single API for
resolveLocalFileSystemURL which needed 2 or 3 APIs (2 or 3 IPCs) before.

These CLs depend on this change:
(Blink) https://codereview.chromium.org/23537011/
(Chromium) https://codereview.chromium.org/23856002/


BUG=177137
TEST=n/a

Review URL: https://chromiumcodereview.appspot.com/23740009

git-svn-id: svn://svn.chromium.org/blink/trunk@157698 bbb929c8-8fbe-4397-9dbb-9b2b20218538