Rename ComposedShadowTreeWalker to ComposedTreeWalker.

The motivation is to use the terminology the shadow DOM spec is using.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23669002

git-svn-id: svn://svn.chromium.org/blink/trunk@156822 bbb929c8-8fbe-4397-9dbb-9b2b20218538