WebStateListObserver methods have default implementation.

As most of the observer are only interested in a sub-set of the
events and to mirror the Objective-C API (WebStateListObserving)
that marks all methods as optional, add empty implementation to
WebStateList methods.

BUG=687207

Review-Url: https://codereview.chromium.org/2708733003
Cr-Commit-Position: refs/heads/master@{#452516}
7 files changed