Fix comma typo in cts tests' entry in gn_isolate_map.pyl

TBR=jbudorick

Bug: 880623
Change-Id: Ibcda8cadf0e6b00f1f50ade12c06922a2f5f3bda
Reviewed-on: https://chromium-review.googlesource.com/c/1332610Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607439}

testing/buildbot/gn_isolate_map.pyl

@@ -1392,7 +1392,7 @@
       "../../android_webview/tools/run_cts.py",
       "--skip-expected-failures",
       "--additional-apk",
-      "apks/SystemWebView.apk"
+      "apks/SystemWebView.apk",
       "-v",
     ],
   },