Consolidate TextIterator's boolean flags into a single enum.

This patch is a straightforward refactoring of TextIterator, which replaces
two boolean flags m_handledNode and m_handledChildren into a single variable.

These two member variables in conjunction represent progress of the iteration
of m_node. Actually, there are only three possible combinations of their values
instead of four: (m_handledNode, m_handledChildren) = (false, false),
(true, false), and (true, true). These three states correspond to the values of
newly introduced enum: HandledNone, HandledNode and HandledChildren.

This patch is in preparation for a later patch which will add more enums to
IterationProgress.

Refactoring only; no behavior change expected.

BUG=244955

Review URL: https://codereview.chromium.org/98603003

git-svn-id: svn://svn.chromium.org/blink/trunk@163048 bbb929c8-8fbe-4397-9dbb-9b2b20218538