Make SVGElement::setCursorImageValue take a const pointer

Fix SVGElement::setCursorImageValue() to take a const
CSSCursorImageValue* instead of just a regular pointer. This is required
pre-work to make StyleBuilderFunctions take const references (since,
when calling into SVGElement::setCursorImageValue from
StyleBuilderFunctions, only a const pointer is available).

BUG=526586

Review-Url: https://codereview.chromium.org/2027113002
Cr-Commit-Position: refs/heads/master@{#397078}
3 files changed