[FastTextAutosizer] Optimize inflate to skip block descendants

This patch optimizes the inflate child walk loop to skip block
descendants, as these blocks will have inflate called on them by
layout. This closely mirrors the current Text Autosizer's
nextInPreOrderSkippingDescendantsOfContainers.

No new tests as this is just an optimization.

BUG=334549
NOTRY=true

Review URL: https://codereview.chromium.org/140603002

git-svn-id: svn://svn.chromium.org/blink/trunk@165499 bbb929c8-8fbe-4397-9dbb-9b2b20218538