scheduler: Convert enums to enum classes

scheduler: Convert enums to enum classes

Convert enums to enum classes to make their purpose clearer and to make it
easier to expose them to the unit tests. Note that the task queue identifier is
maintained as a numeric enum because it serves as an index to the task queue
manager and selector. No functional changes.

Committed: https://crrev.com/16ebe5b1584ec660893f8890f21a6c1d5cab80fe
Cr-Commit-Position: refs/heads/master@{#318248}

[Sami, 2015-02-26 14:45:29]

skyostil@chromium.org changed reviewers:
+ alexclarke@chromium.org, rmcilroy@chromium.org

[alex clarke (OOO till 29th), 2015-02-26 14:54:21]

lgtm

[Sami, 2015-02-26 14:55:29]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-02-26 14:56:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/962633002/20001

[commit-bot: I haz the power, 2015-02-26 16:27:54]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-26 16:28:31]

Patchset 2 (id:??) landed as
https://crrev.com/16ebe5b1584ec660893f8890f21a6c1d5cab80fe
Cr-Commit-Position: refs/heads/master@{#318248}