Revert 157015 "setAttributeNode() does not set the new value to ..."

> setAttributeNode() does not set the new value to an existing attribute if specified attribute is in a different case.
> 
> setAttributeNode() now performs a case-insensitive search for an existing attribute
> and if found, retrieves the index of such an attribute. For setting the value we
> call upon setAttributeInternal() to which we pass both the index as well as the name.
> The name passed to this method is the same as the one passed to the setAttributeNode() API from the webpage and thus can be in any case.
> 
> However, setAttributeInternal() uses this name to get the corresponding existing node.
> Since this retrieval is not case-insensitive, the existing node is not found and thus
> the new value is not set on the existing node.
> We should instead use the passed index and use that to retrieve the existing node.
> 
> Note that obtaining the attribute's value using getAttributeNode() would still
> return the correct value, i.e. the new one.
> 
> Also, this now makes our behavior similar to that of FF and IE.
> 
> BUG= 279193
> R= esprehn@chromium.org,eseidel@chromium.org,ojan@chromium.org
> 
> Review URL: https://chromiumcodereview.appspot.com/22842013

TBR=a.bah@samsung.com

Review URL: https://codereview.chromium.org/23819015

git-svn-id: svn://svn.chromium.org/blink/trunk@157033 bbb929c8-8fbe-4397-9dbb-9b2b20218538