Fix userVisibility() for files dragged into the renderer.

While trying to fix drag-and-drop for JS-created files, I broke
drag-and-drop for real files. This change fixes userVisibility() for
real files dragged into the renderer. This change also renames a couple
of the File::create methods, to clarify their usage.

BUG=398366

Review URL: https://codereview.chromium.org/423283003

git-svn-id: svn://svn.chromium.org/blink/trunk@179285 bbb929c8-8fbe-4397-9dbb-9b2b20218538