Use log dir set in master result reporter

mLogDir may be null during sharded runs, because it is only set for the
master result reporter. This results in many lost log files.

bug:29763746
Change-Id: I0c6c9c960be4d74574001ed81dc3eec6e66a910a
diff --git a/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/result/ResultReporter.java b/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/result/ResultReporter.java
index 1335fc6..0a558cb 100644
--- a/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/result/ResultReporter.java
+++ b/common/host-side/tradefed/src/com/android/compatibility/common/tradefed/result/ResultReporter.java
@@ -486,7 +486,7 @@
     public void testLog(String name, LogDataType type, InputStreamSource stream) {
         // This is safe to be invoked on either the master or a shard ResultReporter
         try {
-            LogFileSaver saver = new LogFileSaver(mLogDir);
+            LogFileSaver saver = new LogFileSaver(mMasterResultReporter.mLogDir);
             File logFile = saver.saveAndZipLogData(name, type, stream.createInputStream());
             info("Saved logs for %s in %s", name, logFile.getAbsolutePath());
         } catch (IOException e) {