Clean up PageBoundaryRule enum.

Clean up PageBoundaryRule enum.

Replace the {Include,Exclude}PageBoundary values with
AssociateWith{Former,Latter}Page, since it was rather confusing what was meant
by "include" or "exclude" in most situations. Also remove default
PageBoundaryRule values from method declarations taking this enum as a
parameter, since there's really no natural default value, and each caller
should be aware of what it's asking for.

Realizing what's actually going on here allowed for some cleanup in
nextPageLogicalTop().

Also made the enum public, since there are public methods taking it as a
parameter.

No behavioral changes intended.

R=leviw@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200438

[leviw_travelin_and_unemployed, 2015-08-12 21:59:42]

The CQ bit was checked by leviw@chromium.org

[leviw_travelin_and_unemployed, 2015-08-12 21:59:42]

lgtm

This makes way more sense to me. Thanks for the cleanup!

[commit-bot: I haz the power, 2015-08-12 22:00:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287113002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287113002/1

[commit-bot: I haz the power, 2015-08-12 23:46:04]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200438