Pass in a dummy USE env var for cros VM tests.

That USE val gates pushing 200_percent.pak to the VM. Without that
resource, chrome can crash. A follow-up CL will fix all these env vars.

TBR=jbudorick@chromium.org

Change-Id: I61e417253f9293639b62f6d295f59bf556084d44
Reviewed-on: https://chromium-review.googlesource.com/1228778
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#591782}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6df796565e5adcf86a64497f48ac65b5a32c4250

chromeos/run_vm_test.py

@@ -312,6 +312,8 @@ class BrowserSanityTest(RemoteTest):
     # line args.
     if not self._test_env.get('GN_ARGS'):
       self._test_env['GN_ARGS'] = 'is_chromeos = true'
+    if not self._test_env.get('USE'):
+      self._test_env['USE'] = 'highdpi'
     self._test_env['PATH'] = (
         self._test_env['PATH'] + ':' + os.path.join(CHROMITE_PATH, 'bin'))