Rename PageLifecycleObserver to PageVisibilityObserver

Now that PageLifecycleObserver has only pageVisibilityChanged(),
it would make more sense to rename the class accordingly.

BUG=610176

Review-Url: https://codereview.chromium.org/2138663003
Cr-Commit-Position: refs/heads/master@{#404623}
20 files changed