StyleSheetCollection renaming

A FIXME followup.

This change renames:
- StlyeSheetCollectionBase -> StyleSheetCollection
- StlyeSheetCollection -> TreeScopeStyleSheetCollection

Then it splits StyleSheetCollection.h/cpp files for each class.

BUG=none
TEST=none
R=haraken,tkent

Review URL: https://codereview.chromium.org/149173014

git-svn-id: svn://svn.chromium.org/blink/trunk@166110 bbb929c8-8fbe-4397-9dbb-9b2b20218538