@page background painting.

In paginated layout, the document background (typically specified on the
BODY or HTML element) is painted on the page border box. Additionally,
@page rules may specify a background property, and this one is painted
over the entire page, including the margin area. This one is painted
before any document background.
See https://drafts.csswg.org/css-page-3/#painting

All document (fragmented) contents are painted by separate paint layers,
i.e. the layer established by the document root (HTML), in addition to
any other top layers. Page containers and border boxes, on the other
hand, do not establish layers, and are painted normally under the paint
layer of the LayoutView. When printing a page, have BoxFragmentPainter
look up the page container fragment of the current page, and paint just
that, with an infinite cull rect.

PhysicalFragment::IsPaginatedRoot() wasn't entirely correct, since it
would also return true for non-paginated documents, as long as we were
printing. Iframes, for instance, are not laid out for pagination, but
printed as-is. With the new callsites, these false positives would cause
trouble. Fix that, and move the function down to PhysicalBoxFragment, to
utilize IsFragmentationContextRoot().

Pre-paint now needs to visit page container and page border box
fragments, in order to set the paint offsets correctly.

Layout still doesn't set the correct offsets for the page border box
(margins are still handled on the outside of Blink), so for now, make
sure that the tests have zero margins.

Bug: 40286153
Change-Id: Ie4808491f1a570b9d223caeea3fa815995989703
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5522582
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298373}