Make CC scrolling/top-controls unit tests create virtual viewport hierarchy.

These tests were not creating the full inner/page_scale/outer hierarchy. This
is problematic since production code has to check for existence of these
layers, making it more complicated. Also, this means the tests aren't as useful
since they're not running on a layer tree that would be used in the real-world.

This CL converts some of these tests to create the virtual viewport hierarchy,
namely:

Inner-Container
  |-Page-Scale
  |---Inner-Scroll
  |-----Outer-Container
  |-------Outer-Scroll

This hierarchy is what is considered "the viewport" and it gets attached to a
root-layer. In production, the root-layer can be the "root transform layer",
used for dev-tools emulation, or the "elastic overscroll layer". In these tests
we create a dummy root-layer. The changes in this CL make it possible to DCHECK
in LayerTreeHostImpl::UpdateViewportContainerSizes for the existence of the
outer viewport without crashing in tests.

BUG=425102
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1251913002

Cr-Commit-Position: refs/heads/master@{#340125}

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