Make buildSpacingAndGlyphsTransform static

Only used within SVGTextChunkBuilder, but does not use any state.

Review URL: https://codereview.chromium.org/361403004

git-svn-id: svn://svn.chromium.org/blink/trunk@177469 bbb929c8-8fbe-4397-9dbb-9b2b20218538