Bug 1264772 - Set chunk-by-runtime option for mochitest-media on android; r=ahal

MozReview-Commit-ID: 3Hf4SH1LO1X

--HG--
extra : rebase_source : 300bfdebf7e2a87e14218300db8198f1ca8f69e4

testing/mozharness/configs/android/androidarm_4_3.py

@@ -119,6 +119,7 @@ config = {
                 "--log-raw=%(raw_log_file)s",
                 "--log-errorsummary=%(error_summary_file)s",
                 "--screenshot-on-fail",
+                "--chunk-by-runtime",
                 "--total-chunks=2",
                 "--subsuite=media",
             ],