[DevTools] Refactor CSS live locations.

- removed CSSLocation._header as it could be gone; turned it to nullable getter.
- only listen to StyleSheet{Added,Removed} events once, not from every live location.
- removed extra layers from CSSWorkspaceBinding.

BUG=none

Review-Url: https://codereview.chromium.org/2693383003
Cr-Commit-Position: refs/heads/master@{#451426}
4 files changed