Don't use the current size in DoGetBestSize() by default.

Don't use the current size in DoGetBestSize() by default.

Doing this means that a window which was made initially large because no other
windows were present or shown will refuse to shrink to make space for them
later even if it doesn't need to be that large. The issue was masked under MSW
and GTK because the first size event under these ports is sent so early that
the top level parent window is not realized yet and has default size of 20*20
pixels meaning that its children will cache their initial (very small) size as
their best size, but it was very visible under OS X where the initial layout
is done using the real top level window size and broke wxInfoBar showing in
the dialogs sample, for example.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62306 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775