[js-code-coverage] Fix v8 coverage output directory

Had the params.task_output_dir as the source of the v8 coverage files,
this is incorrect it should be the tests folder in the
--javascript-coverage-dir flag path.

Bug: 1113941
Change-Id: I8491ce86f6c615be20e7660becedc3c7887cfd13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526805Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Ben Reich <benreich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826186}

testing/merge_scripts/code_coverage/merge_results.py

@@ -103,8 +103,9 @@ def main():
 
     output_path = os.path.join(params.javascript_coverage_dir,
         '%s_javascript.json' % params.merged_js_cov_filename)
+    raw_coverage_folder = os.path.join(params.javascript_coverage_dir, 'tests')
     logging.info('Merging v8 coverage output to %s', output_path)
-    javascript_merger.merge_coverage_files(params.task_output_dir, output_path)
+    javascript_merger.merge_coverage_files(raw_coverage_folder, output_path)
 
   # Name the output profdata file name as {test_target}.profdata or
   # default.profdata.