Rename ScopeContentDistribution to ShadowRootRareData and merge methods into ShadowRoot

Merge methods on ScopeContentDistribution that really belong on ShadowRoot into it
and stop callers from reaching into the internal state of ShadowRoot. Instead there
should be an API. Once those changes were made it became clear that
ScopeContentDistribution is really just rare data for a ShadowRoot so rename the
class as well.

This lets us also shrink all shadow roots that had a ScopeContentDistribution before
since we can move the bitfield for the dirty child insertion point list into the bitfields
on ShadowRoot.

Review URL: https://chromiumcodereview.appspot.com/23539004

git-svn-id: svn://svn.chromium.org/blink/trunk@156874 bbb929c8-8fbe-4397-9dbb-9b2b20218538