[SVG -> OTF Converter] Make placeholders more robust

[SVG -> OTF Converter] Make placeholders more robust
https://bugs.webkit.org/show_bug.cgi?id=139836

Reviewed by Dan Bernstein.

This patch creates a Placeholder object which hides the complexity of
populating offsets to subtables.

No new tests because there is no behavior change.

* svg/SVGToOTFFontConversion.cpp:
(WebCore::SVGToOTFFontConverter::Placeholder::Placeholder):
(WebCore::SVGToOTFFontConverter::Placeholder::populate):
(WebCore::SVGToOTFFontConverter::Placeholder::~Placeholder):
(WebCore::SVGToOTFFontConverter::appendArabicReplacementSubtable):
(WebCore::SVGToOTFFontConverter::appendGSUBTable):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@177620 268f45cc-cd09-0410-ab3c-d52691b4dbfc