[SVG] Make metrics passed to calculateGlyphAdvanceAndOrientation const

Typical in-parameter - the method shouldn't need to mutate this object.

Review URL: https://codereview.chromium.org/392583003

git-svn-id: svn://svn.chromium.org/blink/trunk@178068 bbb929c8-8fbe-4397-9dbb-9b2b20218538