Implement StyleSheetCollectionForDocument and ShadowTreeStyleSheetCollection.

StyleSheetCollectionForDocument and ShadowTreeStyleSheetCollection inherit from StyleSheetCollection.
Each StyleSheetCollection takes care of stylesheets in its treescope.

BUG=244314
TEST=fast/dom/shadow/new-style-inserted-into-parent-treescope.html

Review URL: https://chromiumcodereview.appspot.com/23052002

git-svn-id: svn://svn.chromium.org/blink/trunk@156074 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unused text file extensions from checker.py

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22976003

git-svn-id: svn://svn.chromium.org/blink/trunk@156075 bbb929c8-8fbe-4397-9dbb-9b2b20218538