Fix & clarify some comments in compositing. Rename clippedByAncestor to more accurately
describe what it actually does.

BUG=

Review URL: https://codereview.chromium.org/266473009

git-svn-id: svn://svn.chromium.org/blink/trunk@173017 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make default for deep parameter in importNode false, as specified in:

http://dom.spec.whatwg.org/#document

Update tests to reflect this change. It matches Firefox behavior.

Review URL: https://codereview.chromium.org/252713004

git-svn-id: svn://svn.chromium.org/blink/trunk@173018 bbb929c8-8fbe-4397-9dbb-9b2b20218538