Make ClientRectList.item() argument mandatory and drop [IsIndex]

Make ClientRectList.item() argument mandatory and drop [IsIndex] IDL extended
attribute to match the latest specification:
http://dev.w3.org/csswg/cssom-view/#dom-clientrectlist-item

According to the specification, the index argument for ClientRectList.item() is
mandatory and we should not throw when the index is negative, as long as it
wraps to a valid index value (when calling toUInt32() as per Web IDL
specification).

Our new behavior is consistent with Firefox 22 as well. Note however that IE10
behaves differently: the argument is optional and passing a negative index
throws even if it wraps to a valid index.

Note that according to the specification, we should throw an IndexSizeError
exception when index is greater than the number of ClientRect objects associated
with the object. Blink just returns null in this case and this patch does not
change this behavior as it is consistent with Firefox 22. IE10 does throw in
this case though.

BUG=237739

Review URL: https://chromiumcodereview.appspot.com/22191002

git-svn-id: svn://svn.chromium.org/blink/trunk@155588 bbb929c8-8fbe-4397-9dbb-9b2b20218538