Simplify SVGSVGElement::getElementById method

Simplify SVGSVGElement::getElementByID method code and
also by using getAllElementsById which caches elements,
we can avoid looking for matching element SVG subtree everytime.

Review URL: https://codereview.chromium.org/178323002

git-svn-id: svn://svn.chromium.org/blink/trunk@167890 bbb929c8-8fbe-4397-9dbb-9b2b20218538