Add comments to ElementData.h

Document what ElementData, ShareableElementData and UniqueElementData are.

BUG=334858

Review URL: https://codereview.chromium.org/141593003

git-svn-id: svn://svn.chromium.org/blink/trunk@165960 bbb929c8-8fbe-4397-9dbb-9b2b20218538