Throw exceptions for charnum == getNumberOfChars in get{...}OfChar APIs

Throw exceptions for charnum == getNumberOfChars in get{...}OfChar APIs

According to the spec [1], IndexSizeError should be thrown if:

"...the charnum is negative or if charnum is greater than or equal to
 the number of characters at this node."

This is what's implemented by IE, Gecko and Presto.

[1] http://www.w3.org/TR/SVG11/text.html#__svg__SVGTextContentElement__getStartPositionOfChar
    The current SVG2 draft has a different formulation:

     (https://svgwg.org/svg2-draft/text.html#__svg__SVGTextContentElement__getStartPositionOfChar)
     "If cluster is null, then then throw a DOMException with code
      INDEX_SIZE_ERR."

    but will have the same result.

BUG=391345
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201411

[fs, 2015-08-28 14:51:42]

fs@opera.com changed reviewers:
+ fmalita@chromium.org, pdr@chromium.org

[fs, 2015-08-28 14:51:42]

Interop triviality

[f(malita), 2015-08-28 15:00:54]

lgtm

[fs, 2015-08-28 15:28:34]

The CQ bit was checked by fs@opera.com

[commit-bot: I haz the power, 2015-08-28 15:28:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1321783002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1321783002/1

[commit-bot: I haz the power, 2015-08-28 17:16:26]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201411