[SVG] Clipping non-SVG elements should observe the zoom factor.

When applied to non-SVG elements, SVG clips do not receive a CTM with
the zoom factor included (they are applied outside of any RenderSVGRoot
context).

This CL updates RenderSVGResourceClipper::applyClippingToContext() to
compensate for the missing factor - but only when using userSpaceOnUse
units: for objectBoundingBox units, the zoom is accounted for during
length resolution.

R=pdr@chromium.org,schenney@chromium.org
BUG=272223

Review URL: https://codereview.chromium.org/86463003

git-svn-id: svn://svn.chromium.org/blink/trunk@162650 bbb929c8-8fbe-4397-9dbb-9b2b20218538