web_tests: fix a crash when baselines are missing

This fixes a problem when using run_web_tests.py with --copy-baselines
when the original test expectation does not exist for some reason.

Change-Id: I43fafb982936173639d213a0fa560ef976e13fe8
Reviewed-on: https://chromium-review.googlesource.com/c/1311929
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605940}

third_party/blink/tools/blinkpy/web_tests/controllers/single_test_runner.py

@@ -231,7 +231,11 @@ class SingleTestRunner(object):
             port.update_baseline(output_path, data)
         else:
             _log.info('Copying baseline to "%s"', port.relative_test_filename(output_path))
-            fs.copyfile(current_expected_path, output_path)
+            if fs.exists(current_expected_path):
+                fs.copyfile(current_expected_path, output_path)
+            else:
+                _log.error('Could not copy baseline to "%s" from "%s" because the source file does not exist',
+                           port.relative_test_filename(output_path), current_expected_path)
 
     def _handle_error(self, driver_output, reference_filename=None):
         """Returns test failures if some unusual errors happen in driver's run.