autoroller: Use HEAD instead of LKGR.

autoroller: Use HEAD instead of LKGR.

Since LKGR is sometimes lagging behind, always use HEAD to
minimize the number of commits to bisect when something breaks for us.
As long as the rolls are passing our CQ, we should be fine.

BUG=webrtc:4688
R=phoglund@webrtc.org

Committed: https://crrev.com/18b042f8341a235c39f3818ee5442c6a2bea47e5
Cr-Commit-Position: refs/heads/master@{#10139}

[kjellander_webrtc, 2015-10-01 17:22:16]

kjellander@webrtc.org changed reviewers:
+ phoglund@webrtc.org

[phoglund, 2015-10-02 06:54:44]

lgtm

[kjellander_webrtc, 2015-10-02 07:02:01]

Committed patchset #1 (id:1) manually as
18b042f8341a235c39f3818ee5442c6a2bea47e5 (presubmit successful).

[commit-bot: I haz the power, 2015-10-02 07:02:04]

Patchset 1 (id:??) landed as
https://crrev.com/18b042f8341a235c39f3818ee5442c6a2bea47e5
Cr-Commit-Position: refs/heads/master@{#10139}