Rename variables in ContainerNode::checkForSiblingStyleChanges() for clarity

Rename variables in ContainerNode::checkForSiblingStyleChanges() for clarity
and also update comments.

Variables renaming:
- beforeChange / afterChange to nodeBeforeChange / nodeAfterChange to be clear
  about the type (e.g. not necessary Element nodes)
- newFirstChild / newLastChild to firstChildElement / lastChildElement to be
  clear that those are the first / last child _elements_.
- lastElementBeforeInsertion / firstElementAfterInsertion to
  elementBeforeChange / elementAfterChange as the change is not necessarily an
  insertion (e.g. could be a removal). The naming was confusing.

Also add an ASSERT(changeType != FinishedParsingChildren) assertion to enforce
the precondition explained in the comment for the corresponding scope.

There is no functional change in this CL to make it more easily reviewable.
This CL is purely about improving code readability / understandability.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/380073002

git-svn-id: svn://svn.chromium.org/blink/trunk@178055 bbb929c8-8fbe-4397-9dbb-9b2b20218538