Fixes for SVGPathElement IDLs and a bad assert.

The SVGPathElement IDLs were allowing undefined arguments, optional
arguments, and were not type checking. This patch removes the optional
portion, adds StrictTypeChecking (although that is not implemented in
the IDL compiler yet) and adds testing.

We also remove an assert that is simply wrong. It is OK to try to
commit a change to an SVGPathSegList chunk before it is added to a
list. The role is set from Undefined to Unaltered when it is added
to the list, which is the correct behavior.

R=pdr,fmalita
BUG=311246

Review URL: https://codereview.chromium.org/54893002

git-svn-id: svn://svn.chromium.org/blink/trunk@161130 bbb929c8-8fbe-4397-9dbb-9b2b20218538