[blinkpy] Do not use CWD as output dir on Swarming

The merge_web_test_results.py script used to use the current working
directory when --output-directory isn't specified but --output-json is
(which is the case on Swarming). This could be dangerous as the script
will clean everything in the output directory.

This CL changes the behaviour to always create a temporary output
directory if it's not specified.

Bug: 960994
Change-Id: If1dd0f060557e7fb7014ea0a1dc2ba0bf234e1e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606820Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659717}

third_party/blink/tools/blinkpy/web_tests/merge_results.py

@@ -781,11 +781,6 @@ directory. The script will be given the arguments plus
                 assert (result_key, build_prop_key) in RESULTS_JSON_VALUE_OVERRIDE_WITH_BUILD_PROPERTY, (
                     "%s not in %s" % (result_key, RESULTS_JSON_VALUE_OVERRIDE_WITH_BUILD_PROPERTY))
 
-        if not args.output_directory:
-            args.output_directory = os.getcwd()
-            args.allow_existing_output_directory = True
-            args.remove_existing_output_directory = True
-
         assert not args.input_directories
         args.input_directories = [os.path.dirname(f) for f in args.positional]
         args.positional = []
@@ -797,6 +792,7 @@ directory. The script will be given the arguments plus
 
     if not args.output_directory:
         args.output_directory = tempfile.mkdtemp(suffix='webkit_layout_test_results.')
+        args.allow_existing_output_directory = True
 
     assert args.output_directory
     assert args.input_directories