Fix SitePerProcessMouseWheelHitTestBrowserTest.* tests on Win official.

https://chromium-review.googlesource.com/865558 fixes this flaky tests
on win7_chromium_rel_ng, but it only waits for dispatching of the
first wheel event before sending the others.

This cl makes sure that each wheel event gets dispatched before sending
the next one.

I haven't been able to reproduce the bug on my local windows official
build but I suspect that after using async wheel targeting for wheel
events some of the wheel events in RunTest get coalesced to each other
before getting dispatched.

Bug: 800822
Change-Id: Id693ef3beff19daf08c3e11c2119b2bf90ef6737
Reviewed-on: https://chromium-review.googlesource.com/893803Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533002}

content/browser/site_per_process_hit_test_browsertest.cc

@@ -1600,6 +1600,7 @@ class SitePerProcessMouseWheelHitTestBrowserTest
 
     InputEventAckWaiter waiter(expected_target->GetRenderWidgetHost(),
                                blink::WebInputEvent::kMouseWheel);
+    waiter.Reset();
     SendMouseWheel(pos);
     waiter.Wait();
 
@@ -1609,7 +1610,9 @@ class SitePerProcessMouseWheelHitTestBrowserTest
     EXPECT_TRUE(msg_queue.WaitForMessage(&reply));
     EXPECT_EQ("\"scroll: 1\"", reply);
 
+    waiter.Reset();
     SendMouseWheel(pos);
+    waiter.Wait();
 
     // If async_wheel_events is disabled, this time only the wheel handler
     // fires, since even numbered scrolls are prevent-defaulted. If it is
@@ -1624,7 +1627,9 @@ class SitePerProcessMouseWheelHitTestBrowserTest
       EXPECT_EQ("\"scroll: 2\"", reply);
     }
 
+    waiter.Reset();
     SendMouseWheel(pos);
+    waiter.Wait();
 
     // Odd number of wheels, expect both wheel and scroll handlers to fire
     // again.