video_VEA: fix gtest_filter overwritten in stream loop

The bug was hit if predefined_filter differs between streams. Keep
gtest_filter as argument from commands and don't overwrite it.

BUG=chromium:728906
TEST=manual test

Change-Id: I44babb56ba7b63db666011d93b2057e38dd6bf61
Reviewed-on: https://chromium-review.googlesource.com/541177
Commit-Ready: Pin-chih Lin <johnylin@chromium.org>
Tested-by: Pin-chih Lin <johnylin@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
diff --git a/client/site_tests/video_VideoEncodeAccelerator/video_VideoEncodeAccelerator.py b/client/site_tests/video_VideoEncodeAccelerator/video_VideoEncodeAccelerator.py
index e981f5c..4c2349a 100644
--- a/client/site_tests/video_VideoEncodeAccelerator/video_VideoEncodeAccelerator.py
+++ b/client/site_tests/video_VideoEncodeAccelerator/video_VideoEncodeAccelerator.py
@@ -141,10 +141,11 @@
             if gtest_filter and predefined_filter:
                 logging.warning('predefined gtest filter is suppressed: %s',
                     predefined_filter)
-            if not gtest_filter:
-                gtest_filter = predefined_filter
-            if gtest_filter:
-                cmd_line_list.append('--gtest_filter="%s"' % gtest_filter)
+                applied_filter = gtest_filter
+            else:
+                applied_filter = predefined_filter
+            if applied_filter:
+                cmd_line_list.append('--gtest_filter="%s"' % applied_filter)
 
             cmd_line = ' '.join(cmd_line_list)
             try: