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In the current implementation, sometimes the scope of a page's "pause"
is too wide compared to the definition in the spec [1], and sometimes it
does not pause at all. So narrow the scope to better conform to the
spec, and apply it consistently to all possible ways users can print.

This is based on an earlier CL [2] which only did this for printing with
the system dialog, but not for Print Preview. Some of the trickier cases
handled, which require unpausing, are:
- Navigations while previewing.
- Printing selections, which require a navigation.

[1] https://html.spec.whatwg.org/C/#printing-steps
[2] https://crrev.com/c/1669250

Bug: 956832
Change-Id: I9bf274fcfb70eefe896fbbae9ce13a88b6f4dfc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2338819Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797120}