Require non-zero physical & logical sizes in SVGPreserveAspectRatio::getCTM

This patch fixes a bug where zero physical or logical dimensions could
result in infinite or nan values in SVGPreserveAspectRatio::getCTM.
Several asserts have been added and callers have been updated to handle
the zero case. The CTM will not be properly defined in these cases so
returning the identity matrix is acceptable. This patch is heavily tested
in existing tests.

BUG=415674

Review URL: https://codereview.chromium.org/585623002

git-svn-id: svn://svn.chromium.org/blink/trunk@182293 bbb929c8-8fbe-4397-9dbb-9b2b20218538