[code coverage] Make a temporary dir result dir for layout tests

This CL works around the issue that layout test results merger script
tries to use the profdata directory (CWD) as output directry and wipes
out the content of that directory.

Bug: 960994
Change-Id: I1ca5383d9eb19aba6799cf19d0d5d95c2af82c9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602332Reviewed-by: Roberto Carrillo <robertocn@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658379}

testing/merge_scripts/code_coverage/merge_results.py

@@ -17,6 +17,7 @@ import logging
 import os
 import subprocess
 import sys
+import tempfile
 
 import merge_lib as coverage_merger
 
@@ -82,6 +83,25 @@ def main():
         '--output-json',
         params.output_json,
     ]
+
+    # TODO(crbug.com/960994): Without specifying an output directory, the layout
+    # merge script will use the CWD as the output directory and then tries to
+    # wipe out the content in that directory, and unfortunately, the CWD is a
+    # temporary directory that has been used to hold the coverage profdata, so
+    # without the following hack, the merge script will deletes all the profdata
+    # files and lead to build failures.
+    #
+    # This temporary workaround is only used for evaluating the stability of the
+    # linux-coverage-rel trybot, it should be removed before merging into
+    # linxu-rel as it's not reliable enough, for example, things could break if
+    # the name or arguments of the script are changed.
+    if params.additional_merge_script.endswith('merge_web_test_results.py'):
+      new_args.extend([
+        '--output-directory',
+        tempfile.mkdtemp(),
+        '--allow-existing-output-directory',
+      ])
+
     if params.additional_merge_script_args:
       new_args += json.loads(params.additional_merge_script_args)