Merge ContentDistributor into ElementShadow

ContentDistributor is really part of ElementShadow, many of the methods even make
you pass in the distributor or the host leading to really weird things like
shadow()->distributor().ensureSelectFeatureSet(shadow()). Instead we just merge
them together and the code becomes much more clear.

I also moved ContentDistribution and ScopeContentDistribution to their own files
since they don't need to be in the same file as the distribution process.

Review URL: https://chromiumcodereview.appspot.com/23133006

git-svn-id: svn://svn.chromium.org/blink/trunk@156262 bbb929c8-8fbe-4397-9dbb-9b2b20218538