Bug 1833244 p2. Create infrastructure to pass page dimensions to PrintTarget::BeginPage. r=dholbert

OS print drivers/devices know nothing about page dimensions unless we tell
them. Previously, the physical page dimensions (including orientation) have
always been the same, so communicating their dimensions once at the start of
a print has been enough. In preparation for supporting different "physical"
page dimensions (in the immediate future only different page orientations) when
we save to PDF, we need to have the infrastructure to pass dimensions through
on a page-by-page basis. This patch adds that.

None of the PrintTarget subclasses do anything with this extra information yet,
but in a follow-up patch PrintTargetPDF will use this information to create
PDFs with mixed page orientations.

Differential Revision: https://phabricator.services.mozilla.com/D179423