Make SVGLength store a const CSSValue

Make SVGLength::m_value a Member<const CSSPrimitiveValue> instead of
non-const. This also involved making
Element::addPropertyToPresentationAttributeStyle take a const ptr, and
MutableStylePropertySet::setProperty() take a const ptr.

This is an entirely mechanical change that is more semantically correct
and safe, and is pre-work for making CSSPropertyParser::parseSingleValue
return a const CSSValue*.

BUG=526586

Review-Url: https://codereview.chromium.org/2037323002
Cr-Commit-Position: refs/heads/master@{#405443}

• third_party/WebKit/Source/core/dom/Element.cpp[diff]
• third_party/WebKit/Source/core/dom/Element.h[diff]
• third_party/WebKit/Source/core/svg/SVGLength.h[diff]