Pass references to SVGTextLayoutEngine::layoutTextOnLineOrPath

|text| and |style| are dereferenced in the (single) caller, so pass them
as references instead of pointers.
Also make the reference to the RenderSVGInlineText const, since it
shouldn't be mutated in this context. This in turn requires some additional
const-accessors on RenderSVGInlineText and SVGTextLayoutAttributes.

Review URL: https://codereview.chromium.org/390373004

git-svn-id: svn://svn.chromium.org/blink/trunk@178173 bbb929c8-8fbe-4397-9dbb-9b2b20218538