Make SVGTextContentElement methods behave consistently with Firefox and IE

Make SVGTextContentElement methods behave consistently with Firefox 23 and
IE10:
- We should not throw when negative values are passed in arguments as long
  as they wrap to a valid positive index.
- Arguments should be mandatory

Also fixed a bug in SVGTextContentElement.getSubStringLength() where the
wrong result would be returned if the nchars argument was
too large due to an overflow. To avoid this issue, we clamp the nchars
value to (numberOfChars - charnum) similarly to what is done in
SVGTextContentElement.selectSubString(), and as per the specification:
http://www.w3.org/TR/SVG2/text.html#__svg__SVGTextContentElement__getSubStringLength

BUG=268269

Review URL: https://chromiumcodereview.appspot.com/22798002

git-svn-id: svn://svn.chromium.org/blink/trunk@156036 bbb929c8-8fbe-4397-9dbb-9b2b20218538