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}

tools-webrtc/mb/mb_unittest.py

@@ -448,6 +448,7 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(command, [
         '../../testing/test_env.py',
         '../../tools-webrtc/gtest-parallel-wrapper.py',
+        '--gtest_color=no',
         '--output_dir=${ISOLATED_OUTDIR}/test_logs',
         './base_unittests',
         '--workers=1',
@@ -493,6 +494,7 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(command, [
         '../../testing/xvfb.py',
         '../../tools-webrtc/gtest-parallel-wrapper.py',
+        '--gtest_color=no',
         '--output_dir=${ISOLATED_OUTDIR}/test_logs',
         './base_unittests',
         '--',
@@ -538,6 +540,7 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(command, [
         '../../testing/test_env.py',
         '../../tools-webrtc/gtest-parallel-wrapper.py',
+        '--gtest_color=no',
         '--output_dir=${ISOLATED_OUTDIR}\\test_logs',
         r'.\unittests.exe',
         '--',
@@ -579,6 +582,7 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(command, [
         '../../testing/test_env.py',
         '../../tools-webrtc/gtest-parallel-wrapper.py',
+        '--gtest_color=no',
         '--output_dir=${ISOLATED_OUTDIR}/test_logs',
         './base_unittests',
         '--',