Fix use-of-uninitialized-value in TransformationMatrix::blend()

TransformationMatrix::blend() was attempting to blend between non-invertable
matricies. This resulted in garbage stack variables being used.
This patch ensures that blend() will fall back to a 50% step interpolation
when one of the sides are not invertable.

BUG=389316

Review URL: https://codereview.chromium.org/352793004

git-svn-id: svn://svn.chromium.org/blink/trunk@177453 bbb929c8-8fbe-4397-9dbb-9b2b20218538