Fix flaky AbstractPromiseTest.ThreadHopping

Promises |p2|, |p3| and |p4| could be resolved by another thread by the time
the EXPECT_FALSE is executed on the main thread leading to failures.

This patch removes these to make the test non-flaky.

Bug: 906125
Change-Id: Iab8afc16a9c4532678fe70f4237f1a894a8d5ca1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627350
Auto-Submit: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662605}

base/task/promise/abstract_promise_unittest.cc

@@ -2073,9 +2073,6 @@ TEST_F(AbstractPromiseTest, ThreadHopping) {
 
   p1->OnResolved();
 
-  EXPECT_FALSE(p2->IsResolved());
-  EXPECT_FALSE(p3->IsResolved());
-  EXPECT_FALSE(p4->IsResolved());
   EXPECT_FALSE(p5->IsResolved());
   run_loop.Run();
   EXPECT_TRUE(p2->IsResolved());