This reverts commit 9198ef8e.

Reason for revert: Spec revert: appears to be causing large number of chromium CQ failures on patch application.

Original change's description:
> [bot_update] Default to refs/heads/master
> 
> When revision is not specified, currently HEAD is checked out.
> Change the default to refs/heads/master
> 
> Recipe-Nontrivial-Roll: infra
> Recipe-Nontrivial-Roll: build
> Recipe-Nontrivial-Roll: build_limited_scripts_slave
> Bug: 940214
> Change-Id: I8be59095d90447ea898d5247a6433f0734f5e63b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1554862
> Commit-Queue: Nodir Turakulov <nodir@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

TBR=nodir@chromium.org,tandrii@chromium.org,jbudorick@chromium.org

Change-Id: Icafd817d8f044a4347342bd85dc97d193329a704
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 940214
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1555514Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>