Use better window placement heuristics for internal clients

Currently, we do some sort of window placement only for decorated
internal windows, which feels hacky.

With this change, all internal clients will go through the window
placement code, unless it's a popup or the BypassWindowManagerHint
flag is set.

If the BypassWindowManagerHint flag is set, the window must have
valid position.

CCBUG: 400675


(cherry picked from commit bc347365)