Implement scale*() methods in DOMMatrix.

The methods multiply a scale transformation on the current matrix.

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/450533006

git-svn-id: svn://svn.chromium.org/blink/trunk@180421 bbb929c8-8fbe-4397-9dbb-9b2b20218538