[SVG] Incorrect transform-origin under zoom.

The current SVGGraphicsElement::animatedLocalTransform() tries to work
around pre-scaled CSS length values but it doesn't handle percentage
transform-origin properties correctly: the bounding box used in the CSS
transform calculation is not scaled the same way as the other lengths.

The CL addresses this case and also refactors the workaround: instead of
post-tweaking the translation components directly, compute the CSS
transform under an effective 1.0 zoom factor (as expected under SVG).

BUG=375664
R=ed@opera.com,fs@opera.com

Review URL: https://codereview.chromium.org/305603002

git-svn-id: svn://svn.chromium.org/blink/trunk@174907 bbb929c8-8fbe-4397-9dbb-9b2b20218538