Remove the shouldCollectGarbageSoon flag from V8PerIsolateData

- No one calls setShouldCollectGarbageSoon(), so we can remove it.

- hintForCollectGarbage() always returns false because no one sets the shouldCollectGarbageSoon flag. Thus we can remove hintForCollectGarbage().

BUG=341032

Review URL: https://codereview.chromium.org/179873012

git-svn-id: svn://svn.chromium.org/blink/trunk@168004 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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: svn://svn.chromium.org/blink/trunk@168005 bbb929c8-8fbe-4397-9dbb-9b2b20218538