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Previously, we'd sorted along world-space z which provided a stable
ordering given that the elements did not move in world space in a way
that would upset the element ordering.

However, with viewport-aware elements, this has now changed. With this
change, we instead order the elements semantically. When we add effects
which require sorting by depth (i.e., where elements dynamically change
their ordering), we can augment UiRenderer::GetElementsInDrawOrder to
account for the view vector. Currently, however, this is not required.

Bug: 765460
Change-Id: If17a9b19b35ff6c1e998694f2677cb5c19170d96
Reviewed-on: https://chromium-review.googlesource.com/727342Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510144}