Next attempt to deflake CompilerDispatcherTest

BUG=v8:6069
R=rmcilroy@chromium.org

Change-Id: I0e1096e20fa96af0a4875704f3f90e8458750356
Reviewed-on: https://chromium-review.googlesource.com/456557Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43911}

test/unittests/compiler-dispatcher/compiler-dispatcher-unittest.cc

@@ -722,9 +722,12 @@ TEST_F(CompilerDispatcherTest, FinishNowDuringAbortAll) {
   // Force the compilation to finish, even while aborting.
   ASSERT_TRUE(dispatcher.FinishNow(shared));
   ASSERT_TRUE(dispatcher.jobs_.empty());
-  {
+  for (;;) {
     base::LockGuard<base::Mutex> lock(&dispatcher.mutex_);
+    if (dispatcher.num_background_tasks_ == 0) {
       ASSERT_FALSE(dispatcher.abort_);
+      break;
+    }
   }
 
   ASSERT_TRUE(platform.ForegroundTasksPending());