Update a few JumpList comments

This CL removes some obsolete comments. For example, the class
RecentlyClosedTabsHandler is gone, so the related comments should be
deleted as well.

Other comment updates are according to Google C++ style guide.
https://google.github.io/styleguide/cppguide.html#Comments

BUG=40407, 179576

Review-Url: https://codereview.chromium.org/2849113003
Cr-Commit-Position: refs/heads/master@{#468637}
2 files changed