[SVG] RenderSVGImage may taint its GraphicsContext state.

After r166180, RenderSVGImage::paint() avoids
saving its context state when there is no local transform. But
SVGRenderingContext::prepareToRenderSVGContent() may change the clip
wihout saving, so we must also save/restore when instantiating a
SVGRenderingContext.

BUG=349701
R=schenney@chromium.org

Review URL: https://codereview.chromium.org/192773003

git-svn-id: svn://svn.chromium.org/blink/trunk@168868 bbb929c8-8fbe-4397-9dbb-9b2b20218538