Change the type of TextIteratorBehavior from enum to unsigned int.

TextIteratorBehavior is originally defined as enum, but that is inconvenient
since it is actually a bit mask and we need to use static_cast<> to pass
a value consisting of multiple behaviors.

This simple patch adds a new typedef called TextIteratorBehaviorFlags defined
as unsigned int so we can pass combined flags to TextIterator APIs easily.

This is a follow-up of <https://codereview.chromium.org/100433007>.

BUG=none

Review URL: https://codereview.chromium.org/101653004

git-svn-id: svn://svn.chromium.org/blink/trunk@163774 bbb929c8-8fbe-4397-9dbb-9b2b20218538