Added sandbox_linux_unittests to ARM trybot

Added sandbox_linux_unittests test to
linux_arm_cross_compile/linux_arm_tester builders on the
tryserver.chromium waterfall.

R=phajdan.jr@chromium.org
BUG=386445
NOTRY=true

Review URL: https://codereview.chromium.org/342943007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278626 0039d316-1c4b-4281-b951-d872f2087c98

testing/buildbot/chromium_arm.json

 {
   "compile_targets": [
-    "browser_tests_run"
+    "browser_tests_run",
+    "sandbox_linux_unittests_run"
   ],
   "gtest_tests": [
-    "browser_tests"
+    "browser_tests",
+    "sandbox_linux_unittests"
   ]
 }