[telemetry] WebComponents Cleanups

[telemetry] WebComponents Cleanups

This is a set of minor cleanups in preparation for adding values that are
web components to telemetry.values.

- Viewer->WebComponent because we'll have telemetry.values.web_component_value
- Let WebComponent data_to_view be mutable
- Allow module dependencies of a WebComponent be computed by the subclass,
  in case it does data-dependent class loading [which they will need to do].

R=dtu
BUG=340096
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260044

[nduca, 2014-03-27 02:00:33]

ptal

[dtu, 2014-03-27 20:04:31]

lgtm

[nduca, 2014-03-27 20:09:52]

The CQ bit was checked by nduca@chromium.org

[commit-bot: I haz the power, 2014-03-27 20:12:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nduca@chromium.org/214143002/1

[commit-bot: I haz the power, 2014-03-27 23:47:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-27 23:47:59]

Commit queue rejected this change because the description was changed
between the time the change entered the commit queue and the time it
was ready to commit. You can safely check the commit box again.

[nduca, 2014-03-28 00:38:16]

The CQ bit was checked by nduca@chromium.org

[commit-bot: I haz the power, 2014-03-28 00:39:12]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/nduca@chromium.org/214143002/1

[commit-bot: I haz the power, 2014-03-28 00:49:34]

Change committed as 260044