Massage the CodeStub class hierarchy a bit.

Massage the CodeStub class hierarchy a bit.

Several tiny changes for the upcoming TurboFan-genrated handlers/stubs:

   * Relaxed the type of code_stub().

   * Made GetCodeKind() public, it effectly was like this, anyway.

   * Const-corrected GetStubType().

Committed: https://crrev.com/6e4e43b652a43d54b2bf51777e781b8f91b251b6
Cr-Commit-Position: refs/heads/master@{#26542}

[Sven Panne, 2015-02-10 10:48:40]

svenpanne@chromium.org changed reviewers:
+ dcarney@chromium.org

[dcarney, 2015-02-10 10:55:40]

lgtm

[Sven Panne, 2015-02-10 11:24:41]

The CQ bit was checked by svenpanne@chromium.org

[commit-bot: I haz the power, 2015-02-10 11:24:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/915583002/1

[commit-bot: I haz the power, 2015-02-10 11:46:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-10 11:46:14]

Patchset 1 (id:??) landed as
https://crrev.com/6e4e43b652a43d54b2bf51777e781b8f91b251b6
Cr-Commit-Position: refs/heads/master@{#26542}