Don't display colors on gtest-parallel logs on swarming.

It makes the logs hard to read, and swarming doesn't output colors anyway.

Also:
- Fix a bug where we tried to access output_dir when combining the logs even if the flag was not set.
- Update a comment explaining why we just 'eat' the --isolated-script-test-chartjson-output flag.

NOTRY=True
BUG=webrtc:7524

Review-Url: https://codereview.webrtc.org/2843263005
Cr-Commit-Position: refs/heads/master@{#17973}
diff --git a/tools-webrtc/mb/mb.py b/tools-webrtc/mb/mb.py
index 7361e12..9d568ce 100755
--- a/tools-webrtc/mb/mb.py
+++ b/tools-webrtc/mb/mb.py
@@ -1096,6 +1096,7 @@
         output_dir = '${ISOLATED_OUTDIR}' + sep + 'test_logs'
         gtest_parallel_wrapper = [
             '../../tools-webrtc/gtest-parallel-wrapper.py',
+            '--gtest_color=no',
             '--output_dir=%s' % output_dir,
         ]