android: Don't print link to logdog stream during tests if it's empty.

See https://chromium-swarm-dev.appspot.com/task?id=39bce1ee054c5e10

R=jbudorick@chromium.org

Bug: 
Change-Id: If635938d30fbe2b527ef972d5c4896b599f42642
Reviewed-on: https://chromium-review.googlesource.com/764575
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515796}

build/android/pylib/local/device/local_device_gtest_run.py

@@ -473,7 +473,8 @@ def _RunTest(self, device, test):
                   timeout=timeout, retries=0)
           logmon.Close()
 
-        logging.info('Logcat saved to %s', logcat_file.Link())
+        if logcat_file.Link():
+          logging.info('Logcat saved to %s', logcat_file.Link())
 
         if self._test_instance.enable_xml_result_parsing:
           try:

build/android/pylib/local/device/local_device_instrumentation_test_run.py

@@ -459,7 +459,8 @@ def name_and_timeout(t):
                 target, raw=True, extras=extras, timeout=timeout, retries=0)
       logmon.Close()
 
-    logging.info('Logcat saved to %s', logcat_file.Link())
+    if logcat_file.Link():
+      logging.info('Logcat saved to %s', logcat_file.Link())
 
     duration_ms = time_ms() - start_ms