Hitting an assert while attempting a medium-sized rebaseline.

What steps will reproduce the problem?

(1) patch https://chromium-review.googlesource.com/c/chromium/src/+/1377519/10 locally
(2) rebaseline-cl --patchset=10

What happens instead?

third_party/blink/tools/blink_tool.py rebaseline-cl --patchset=10
2018-12-20 11:53:57,004 - Finished try jobs found for all try bots.
2018-12-20 11:54:16,208 - No retry summary available for "mac10.13_retina-blink-rel".
2018-12-20 11:54:17,297 - No retry summary available for "mac10.10-blink-rel".
2018-12-20 11:54:17,697 - No retry summary available for "mac10.12-blink-rel".
2018-12-20 11:54:18,497 - No retry summary available for "mac10.13-blink-rel".
2018-12-20 11:54:18,913 - No retry summary available for "mac10.11-blink-rel".
2018-12-20 11:54:21,068 - Rebaselining compositing/contents-opaque/overflow-hidden-child-layers.html
2018-12-20 11:54:21,068 - Rebaselining compositing/culling/scrolled-within-boxshadow.html
...
...
2018-12-20 11:54:21,077 - Rebaselining virtual/video-surface-layer/media/video-no-audio.html
2018-12-20 11:54:21,077 - Rebaselining virtual/video-surface-layer/media/video-zoom-controls.html
2018-12-20 12:00:27,294 - Traceback (most recent call last):
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blink_tool.py", line 83, in <module>
    main()
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blink_tool.py", line 78, in main
    BlinkTool(os.path.abspath(__file__)).main()
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/blink_tool.py", line 105, in main
    result = command.check_arguments_and_execute(options, args, self)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/command.py", line 110, in check_arguments_and_execute
    return self.execute(options, args, tool) or 0
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/optimize_baselines.py", line 41, in execute
    self._optimize_baseline(optimizer, test_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/optimize_baselines.py", line 28, in _optimize_baseline
    optimizer.optimize(test_name, suffix)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 88, in optimize
    succeeded &= self._optimize_subtree(test_name, non_virtual_baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 122, in _optimize_subtree
    results_by_directory, new_results_by_directory = self._find_optimal_result_placement(test_name, baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 401, in _find_optimal_result_placement
    results_by_directory = self.read_results_by_directory(test_name, baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 114, in read_results_by_directory
    results_by_directory[directory] = ResultDigest(self._filesystem, path, self._is_reftest(test_name))
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 512, in __init__
    assert fs.exists(path)
AssertionError

2018-12-20 12:00:27,294 - Traceback (most recent call last):
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blink_tool.py", line 83, in <module>
    main()
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blink_tool.py", line 78, in main
    BlinkTool(os.path.abspath(__file__)).main()
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/blink_tool.py", line 105, in main
    result = command.check_arguments_and_execute(options, args, self)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/command.py", line 110, in check_arguments_and_execute
    return self.execute(options, args, tool) or 0
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/optimize_baselines.py", line 41, in execute
    self._optimize_baseline(optimizer, test_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/tool/commands/optimize_baselines.py", line 28, in _optimize_baseline
    optimizer.optimize(test_name, suffix)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 88, in optimize
    succeeded &= self._optimize_subtree(test_name, non_virtual_baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 122, in _optimize_subtree
    results_by_directory, new_results_by_directory = self._find_optimal_result_placement(test_name, baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 401, in _find_optimal_result_placement
    results_by_directory = self.read_results_by_directory(test_name, baseline_name)
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 114, in read_results_by_directory
    results_by_directory[directory] = ResultDigest(self._filesystem, path, self._is_reftest(test_name))
  File "/mnt/data/home/mali/src/chromium/src/third_party/blink/tools/blinkpy/common/checkout/baseline_optimizer.py", line 512, in __init__
    assert fs.exists(path)
AssertionError