(Cleanup) Implement WebDOMFileSystem::create() etc to deprecate FileSystem-related methods in WebFrame

We have several FileSystem-related scripting methods in WebFrame:
- WebFrame::createFileSystem
- WebFrame::createSerializableFileSystem
- WebFrame::createFileEntry

But they are totally unrelated to Frame and would be better be
isolated into FileSystem-related interface.

This patch adds euivalent methods to WebDOMFileSystem to deprecate them.

BUG=none
TEST=no functional changes

Review URL: https://codereview.chromium.org/179373002

git-svn-id: svn://svn.chromium.org/blink/trunk@167757 bbb929c8-8fbe-4397-9dbb-9b2b20218538