To reproduce:
1. Create a local branch;
2. Make any change to the code;
3. Commit and upload the CL;
4. run "webkit-patch rebaseline-cl"

Expected: It schedules try jobs

Actual: It crashes:

$ webkit-patch rebaseline-cl
2017-10-19 10:14:59,839 - No finished try jobs.
2017-10-19 10:14:59,840 - Triggering try jobs:
2017-10-19 10:14:59,840 -   android_blink_rel
2017-10-19 10:14:59,840 -   linux_trusty_blink_rel
2017-10-19 10:14:59,840 -   mac10.10_blink_rel
2017-10-19 10:14:59,840 -   mac10.11_blink_rel
2017-10-19 10:14:59,840 -   mac10.11_retina_blink_rel
2017-10-19 10:14:59,840 -   mac10.12_blink_rel
2017-10-19 10:14:59,840 -   win10_blink_rel
2017-10-19 10:14:59,840 -   win7_blink_rel
Traceback (most recent call last):
  File "/usr/local/google/home/wangxianzhu/chrome/src/third_party/WebKit/Tools/Scripts/webkit-patch", line 84, in <module>
    main()
  File "/usr/local/google/home/wangxianzhu/chrome/src/third_party/WebKit/Tools/Scripts/webkit-patch", line 79, in main
    WebKitPatch(os.path.abspath(__file__)).main()
  File "/ssd/work/chrome/src/third_party/WebKit/Tools/Scripts/webkitpy/tool/webkit_patch.py", line 114, in main
    result = command.check_arguments_and_execute(options, args, self)
  File "/ssd/work/chrome/src/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/command.py", line 110, in check_arguments_and_execute
    return self.execute(options, args, tool) or 0
  File "/ssd/work/chrome/src/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py", line 102, in execute
    self.trigger_try_jobs(builders_with_no_jobs)
  File "/ssd/work/chrome/src/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py", line 176, in trigger_try_jobs
    self.git_cl.trigger_try_jobs(builders)
  File "/ssd/work/chrome/src/third_party/WebKit/Tools/Scripts/webkitpy/common/net/git_cl.py", line 58, in trigger_try_jobs
    builders.remove('android_blink_rel')
AttributeError: 'frozenset' object has no attribute 'remove'

The second run succeeds:
$ webkit-patch rebaseline-cl
2017-10-19 10:16:45,778 - No finished try jobs.
2017-10-19 10:16:45,779 - Scheduled or started try jobs:
2017-10-19 10:16:45,779 -   android_blink_rel
2017-10-19 10:16:45,779 - Triggering try jobs:
2017-10-19 10:16:45,779 -   linux_trusty_blink_rel
2017-10-19 10:16:45,779 -   mac10.10_blink_rel
2017-10-19 10:16:45,779 -   mac10.11_blink_rel
2017-10-19 10:16:45,780 -   mac10.11_retina_blink_rel
2017-10-19 10:16:45,780 -   mac10.12_blink_rel
2017-10-19 10:16:45,780 -   win10_blink_rel
2017-10-19 10:16:45,780 -   win7_blink_rel
2017-10-19 10:16:47,210 - Once all pending try jobs have finished, please re-run
webkit-patch rebaseline-cl to fetch new baselines.