This change goes hand in hand with the corresponding Rietveld change here:
https://codereview.chromium.org/773083004/
The motivation for both CLs came from the discussion in the internal CL: https://chromereviews.googleplex.com/115567013/

AFAIK either change can be submitted first without breaking anything in the other framework.

Observe the "Tracked Ref" field in the below CLs-

Tracking a remote ref:
* https://skia-codereview-staging.appspot.com/8861001 (Tracking skiabot-test's refs/heads/master)
* https://skia-codereview-staging.appspot.com/851002 (Tracking skiabot-test's refs/diff/test1)
* https://skia-codereview-staging.appspot.com/2891001 (Tracking Chromium's refs/heads/master)
* https://skia-codereview-staging.appspot.com/1931003 (Tracking Chromium's refs/branch-heads/1916)

Tracking a local branch which in turn tracks a remote ref:
* https://skia-codereview-staging.appspot.com/3891002 (Transitively tracking skiabot-test's refs/heads/master)
* https://skia-codereview-staging.appspot.com/4921001 (Transitively tracking Chromium's refs/branch-heads/1916)

CL when no target_ref is specified in depot_tools/third_party/upload.py:
* https://skia-codereview-staging.appspot.com/3871003 (CL with missing target_ref should default to /refs/heads/master)


Try the above links with and without the 'Deprecated UI' checked in https://skia-codereview-staging.appspot.com/settings


BUG=435702

Review URL: https://codereview.chromium.org/781523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293334 0039d316-1c4b-4281-b951-d872f2087c98