Set default font size to 10px as per spec, not 10pt.

See: http://www.w3.org/TR/2dcontext/#dom-context-2d-font

Also removes duplicated assignments that are done as part of the
default constructor.

Task-number: QTBUG-31716

Change-Id: I5bd131c14a5defd422d858f02974c215de7adae9
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>