[SVG] Remove redundant identity checks

Minor cleanup: AffineTransform::mapRect() is already optimized for the
identity/translation case, no need to check explicitly.

R=pdr@chromium.org,schenney@chromium.org

Review URL: https://codereview.chromium.org/175763002

git-svn-id: svn://svn.chromium.org/blink/trunk@168031 bbb929c8-8fbe-4397-9dbb-9b2b20218538