Autoroll: Update checkout before reading chromium_revision.

Autoroll: Update checkout before reading chromium_revision.

Ensure the checkout is updated _before_ reading the current
revision from the DEPS file. This will avoid errors when a checkout
is not up to date, as updating happened after reading the revision
before this change.

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

Committed: https://crrev.com/44d5d7ba42d5d595b58a1f7767d50570d4ae5791
Cr-Commit-Position: refs/heads/master@{#10071}

[kjellander_webrtc, 2015-09-24 11:38:28]

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

[phoglund, 2015-09-25 10:36:03]

lgtm

[kjellander_webrtc, 2015-09-25 11:22:03]

Committed patchset #1 (id:1) manually as
44d5d7ba42d5d595b58a1f7767d50570d4ae5791 (presubmit successful).

[commit-bot: I haz the power, 2015-09-25 11:22:06]

Patchset 1 (id:??) landed as
https://crrev.com/44d5d7ba42d5d595b58a1f7767d50570d4ae5791
Cr-Commit-Position: refs/heads/master@{#10071}