Implement file downloads for Chrome HTTP stack (Java side)

This just adds an extra entrypoint to BrowserFrame. It delegates
directly to the identical method in CallbackProxy, and from there
downloads are handled in the same way as before.

See matching C++ change in external/webkit (I81300400)

Change-Id: I87e71debe475d0d24e13a5763fbe803e3cd78cf5
3 files changed