Rename omaha_client and similar tokens to update_client in all contexts.

This is a mechanical change.

The idea here is to refactor the common code involved in installing
components and extensions in a common module. In the future, we want
more update-related Chrome stuff to go in this module.
Therefore, it is desirable that this module have a name that describes
its purpose.

BUG=445949

Review URL: https://codereview.chromium.org/803313003

Cr-Commit-Position: refs/heads/master@{#310022}
57 files changed