Use GuestView embedder when determing the print preview dialog dimensions.
The print preview dialog uses the initiator WebContents to determine its
size. For prints from embedded GuestViews, this can cause print preview
dialogs to be smaller than normal. This CL fixes this by changing
PrintPreviewDialogDelegate::GetDialogSize() to follow the chain of
GuestView embedders to find the outermost WebContents and use it to
determine the dialog size.
BUG=450564
Review URL: https://codereview.chromium.org/
867493002
Cr-Commit-Position: refs/heads/master@{#312545}