Support uploading files larger 2GB via drag and drop and file input (#11947).

Vaadin mostly just passes through the value from the Javascript File object
on drag and drop. As per specification (http://www.w3.org/TR/file-upload/#blob)
the size value can be "long". The size attribute of Html5File was already long,
but this patch makes sure long values actually are transported correctly from
the client side Transferable. Additionally, the file upload handling in
FileUploadHandler was determining the length of the upload via the Servlet Spec
getContentLength method of the request. However the spec was written at a time
where 2GB were unthinkable. Luckily containers return this value by querying
the Content-Length header, which can transport values of "long". So instead of
using the Servlet Spec method, FileUploadHandler evaluates the header itself.
AbstractStreamingEvent already was supporting "long" values. For Drag&Drop,
the file size needs to be serialized as double, because long is not supported
in JSNI.

Change-Id: I606fca7430c65c20144793fa011cb2f6ee3a0415