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This change makes the test more resilient to unexpected state on the
machine.

- RegistryVerifier can now verify that registry values are not present
  (and, as a result, optional registry keys). A key's "exists" property
  can be one of "required", "forbidden", or "optional". The first two
  behave identically to the true and false boolean values of the
  previous implementation. For an "optional" key, values within it may
  have an empty dict (i.e., with no "type" or "data" members) to
  indicate that the value must not be present in the key if the key is
  present.

- Each test now begins with the "no_pv" state rather than the "clean"
  state. This is more permissive, and simply ensures that Chrome is not
  registered with Google Update.

- After attempting to force-uninstall Chrome (either before executing a
  test or after a test failure), the harness will unconditionally delete
  the Google Update registration of that Chrome if it is present. This
  will clean the machine sufficiently for a subsequent test run.

BUG=375739,401148,402081
R=robertshield@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/520633005

Cr-Commit-Position: refs/heads/master@{#292943}