Add disallowPositionReuse

[liberato-at-chromium]

A UA is free to select a position and size for a document PiP window that does not conform to the size requested by the site with requestWindow(). A typical use is to allow user resizes to take precedence if a PiP window is closed and reopened later by the same site, so that the user's preference takes precedence.

This change adds the disallowPositionReuse flag to notify the user agent that this PiP window is semantically unrelated to the previous one, so ignoring the most recent user-selected size might be more appropriate, as if this were the first PiP window opened by the site.