Lazily allocate svg shape transforms, shrink RenderSVGShape by 44B

This patch switches to lazily allocating transforms in RenderSVGShape
because local transforms are relatively rare.

BUG=429551

Review URL: https://codereview.chromium.org/701433002

git-svn-id: svn://svn.chromium.org/blink/trunk@185118 bbb929c8-8fbe-4397-9dbb-9b2b20218538