Activate Network Traffic Annotations trybot test on Linux.

After fixing the following two files, the trybot test on Linux are
reactivated:
'tools/clang/pylib/clang/compile_db.py'
'tools/traffic_annotation/scripts/check_annotations.py'

Bug: 690323
Bug: 807146
Change-Id: Id769e4bb4e5d416eb04e2436d38cec5af04606ec
Reviewed-on: https://chromium-review.googlesource.com/891960
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533218}

tools/traffic_annotation/scripts/check_annotations.py

@@ -17,7 +17,7 @@ import sys
 # If this test starts failing, please set TEST_IS_ENABLED to "False" and file a
 # bug to get this reenabled, and cc the people listed in
 # //tools/traffic_annotation/OWNERS.
-TEST_IS_ENABLED = False
+TEST_IS_ENABLED = sys.platform != 'win32'
 
 
 class NetworkTrafficAnnotationChecker():