Implement beforeprint/afterprint events.

https://html.spec.whatwg.org/multipage/webappapis.html#windoweventhandlers
Also, this CL adds comments about OOPIF printing.


Bug: 218205, 455764
Change-Id: I8f1e7ac200abaf72cc3c104c8690b67c8529098c
Reviewed-on: https://chromium-review.googlesource.com/683856
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504628}
23 files changed