update_engine: Change the default autest Omaha URL

Change the default autest command's default URL to:
https://omaha-qa.sandbox.google.com/service/update2

BUG=chromium:887004
TEST=none

Change-Id: Ib2414162a7f3221b6959b701e83fef27e9cc7e8f
Reviewed-on: https://chromium-review.googlesource.com/1257820
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
(cherry picked from commit 1aafbfdc30b20d9d6a193a0b992234db710cf3c5)

common/platform_constants_chromeos.cc

diff --git a/common/platform_constants_chromeos.cc b/common/platform_constants_chromeos.cc
index 3ebcf8a..3653432 100644
--- a/common/platform_constants_chromeos.cc
+++ b/common/platform_constants_chromeos.cc
@@ -22,7 +22,7 @@
 const char kOmahaDefaultProductionURL[] =
     "https://tools.google.com/service/update2";
 const char kOmahaDefaultAUTestURL[] =
-    "https://omaha.sandbox.google.com/service/update2";
+    "https://omaha-qa.sandbox.google.com/service/update2";
 const char kOmahaUpdaterID[] = "ChromeOSUpdateEngine";
 const char kOmahaPlatformName[] = "Chrome OS";
 const char kUpdatePayloadPublicKeyPath[] =