Refactor delta/sent_delta logic into self-contained abstraction.
This patch updates page scale to use a new SyncedProperty abstraction. This
is shared between the pending and active trees and contains all page scale state
on the impl side except for the min/max limits. Its API only exposes those
operations that are encouraged: for example, the raw main-thread originating value
can no longer be used by accident.
This patch should be a no-op. In future patches, I intend to make scroll offsets
and top controls use this abstraction as well.
NOTRY=true
BUG=
Review URL: https://codereview.chromium.org/
764483002
Cr-Commit-Position: refs/heads/master@{#306736}