Merge two functions, whose names are confusing, into one function in ComposedTreeWalker.

Both ComposedTreeWalker::traverseParentInCurrentTree and ComposedTreeWalker::traverseParentBackToYoungerShadowRootOrShadowHost() are super confusing names.
We don't have much benefits from these two separated functions.

Let's merge them into one function, called ComposedTreeWalker::traverseParentOrHost().

BUG=None
Test=None; No behaviour change;

Review URL: https://codereview.chromium.org/61623002

git-svn-id: svn://svn.chromium.org/blink/trunk@161484 bbb929c8-8fbe-4397-9dbb-9b2b20218538