Switch WebFrameClient::openFileSystem to Platform::openFileSystem

chrome side patch: https://codereview.chromium.org/22666004/

BUG=257349

Review URL: https://chromiumcodereview.appspot.com/22669002

git-svn-id: svn://svn.chromium.org/blink/trunk@156142 bbb929c8-8fbe-4397-9dbb-9b2b20218538