Hoist "layout size changed" check out of SVGLayoutSupport::layoutChildren

For LayoutSVGRoot, this predicate can be computed quite easily, so
calling layoutSizeOfNearestViewportChanged() will be an unnecessary
detour.
Also rearrange layoutSizeOfNearestViewportChanged() to avoid an
unnecessary virtual call.

BUG=603956

Review URL: https://codereview.chromium.org/1902073002

Cr-Commit-Position: refs/heads/master@{#388332}
5 files changed