Fix unclickable PDF links to targets inside OOFs.

Internal links in PDFs generated by the browser were unclickable if the
link's target was inside an absolutely positioned element, which in turn
was inside a relative-positioned container on a subsequent page.

This was caused by OffsetFromRootFragmentationContext not being set on
page area fragments. This needs to be set, since OOFs inside the page
area will use the offset to calculate the correct location of the OOF.

Bug: 437456046
Change-Id: Ie6a106703547476575cba4f7129be470de17776b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6844197
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502575}