As the spec mentions, if the content or layout of the document changes (e.g.
content is added, moved, deleted, resized) such that the content of a snapport
changes, the UA must re-evaluate the resulting scroll position, and re-snap if
required. If the scroll container was snapped before the content change and that
same snap position still exists (e.g. its associated element was not deleted),
the scroll container must be re-snapped to that same snap position after the
content change.

This patch adds SnapForAllContainers() right after UpdateAllSnapContainerData()
during PerformPostLayoutTasks() and adds tests for that.

Some scroll snap tests assume that a scroller's initial scroll position is (0,0)
even with one and only non-zero snap position. This assumption won't hold true
after this change since the scroller would snap at the snap position at the
beginning. So we add an anchor snap element at (0,0) to ensure the scroller
always starts at (0,0) for the tests.

This patch also fixes a bug in VisualViewport::MaximumScrollOffset().
Bug: 866127
Change-Id: Ib050ef91b04d4391a846eda3dbf667e4d412cb08