Rename functions and members in ShadowRoot and ShadowRootRareData.

1. Use 'descendant' rather than 'child'.
   Some functions are actually used to track descendant elements rather than direct child elements of the shadow root.
2. Prefix 'did' to some functions.
   Using 'did' looks an established convention for this kind of functions.
3. Other renaming for consistency.

BUG=None
Test=None; No behavior change;

Review URL: https://codereview.chromium.org/52073002

git-svn-id: svn://svn.chromium.org/blink/trunk@160915 bbb929c8-8fbe-4397-9dbb-9b2b20218538