[code coverage] import jinja2 from third_party

An user reported that running coverage_test.py fails due to missing
jinja2, this CL fixes the problem by importing it from third_party/.

Bug: 941573
Change-Id: I1efdd45424668fcd46b091eccde484fdf81ecaa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1521795
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640853}

tools/code_coverage/coverage_test.py

@@ -8,8 +8,16 @@
 import re
 import shutil
 import subprocess
+import sys
 import unittest
 
+# Appends third_party/ so that coverage_utils can import jinja2 from
+# third_party/, note that this is not done inside coverage_utils because
+# coverage_utils is also used outside of Chromium source tree.
+sys.path.append(
+    os.path.join(
+        os.path.dirname(__file__), os.path.pardir, os.path.pardir,
+        'third_party'))
 import coverage_utils