Text API does not behave according to specification

1. Make Text.splitText() argument mandatory and stop throwing if the offset is
negative as per the specification:
http://www.w3.org/TR/2012/WD-dom-20121206/#dom-text-splittext

This behavior is consistent with IE10 and Firefox.

2. Make Text.replaceWholeText() argument mandatory as per the DOM3 specification:
http://www.w3.org/TR/DOM-Level-3-Core/core.html#Text3-replaceWholeText

This behavior is consistent with IE10. This function is not supported by Firefox,
likely because it was removed from DOM4.

BUG=267509

Review URL: https://chromiumcodereview.appspot.com/21816003

git-svn-id: svn://svn.chromium.org/blink/trunk@155513 bbb929c8-8fbe-4397-9dbb-9b2b20218538