Eliminate recursion in SVGTextMetricsBuilder::walkTree
Eliminate recursion in SVGTextMetricsBuilder::walkTree

This tree-traveral can be trivially rewritten as a pre-order traversal
using RenderObject::nextInPreOrderAfterChildren.

Also fold the branches in the RenderSVGInlineText case to reduce the
number of callsites to measureTextRenderer.

Review URL: https://codereview.chromium.org/175323002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@168005 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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