Rename getNodeInShadowTreeStack to getNodeInTreeOfTrees to utilize the term used in the Shadow DOM spec.

BUG=None
R=morrita@chromium.org, yutak@chromium.org

Review URL: https://codereview.chromium.org/22300016

git-svn-id: svn://svn.chromium.org/blink/trunk@155739 bbb929c8-8fbe-4397-9dbb-9b2b20218538