Do not fail "git cl format" when google-java-format.jar is missing

The file is conditionally downloaded in chrome, and we do not want to
have "git cl format" fail when editing a .java file on non-linux /
non-android checkouts.

Bug: 1462204
Change-Id: I5fc3b74484e1d9304089118449678acb506d5831
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4943676
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 406db64..e3d4e4e 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -6138,7 +6138,9 @@
 
         path = os.path.join(primary_solution_path, 'third_party',
                             'google-java-format', 'google-java-format')
-        if os.path.exists(path):
+        # Check that the .jar exists, since it is conditionally downloaded via
+        # DEPS conditions.
+        if os.path.exists(path) and os.path.exists(path + '.jar'):
             return path
     return None