Add toFloat*Array() methods and use them in layout tests.

The methods return the serialized 16 elements m11 to m44 of the current
matrix in column-major order.

The specification:
  http://dev.w3.org/fxtf/geometry/

Intent to Implement:
  https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V_bJNtOg0oM

BUG=388780

Review URL: https://codereview.chromium.org/483633002

git-svn-id: svn://svn.chromium.org/blink/trunk@180827 bbb929c8-8fbe-4397-9dbb-9b2b20218538