test_env.py: Disable sandbox on clang/win for now.

Swarming uses the command baked into the isolate (via test_env.py) for running
test instead of using the runtest.py script.  Hence, test_env.py needs to
learn everything runtest.py already knows -- for example, that tests can't
run with the sandbox in asan/win yet.

BUG=451741,382867
TBR=scottmg@chromium.org

Review URL: https://codereview.chromium.org/882913004

Cr-Commit-Position: refs/heads/master@{#313452}

testing/test_env.py

@@ -126,6 +126,9 @@ def run_executable(cmd, env):
 
   if asan:
     extra_env.update(get_asan_env(cmd, lsan))
+    # ASan is not yet sandbox-friendly on Windows (http://crbug.com/382867).
+    if sys.platform == 'win32':
+      cmd.append('--no-sandbox')
   if lsan:
     cmd.append('--no-sandbox')