Disable SandboxTest.SpawnSandboxTarget from chrome_cleaner_unittests

It's flaky.

TBR=joenotcharles@chromium.org

Bug: 874387
Change-Id: I3056f49cb1879a6fbacd195f94dfe6992cfd6710
Reviewed-on: https://chromium-review.googlesource.com/1175123Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583209}

chrome/chrome_cleaner/ipc/sandbox_unittest.cc

@@ -162,7 +162,8 @@ MULTIPROCESS_TEST_MAIN(MockSandboxProcessMain) {
 
 }  // namespace
 
-TEST_F(SandboxTest, SpawnSandboxTarget) {
+// Flaky; see http://crbug.com/874387
+TEST_F(SandboxTest, DISABLED_SpawnSandboxTarget) {
   if (base::win::GetVersion() < base::win::VERSION_WIN8) {
     // TODO(b/871924): This test is currently failing on win7. Fix and enable.
     return;