[VizHitTesting] Deflake SitePerProcessHitTestBrowserTest.ScrolledNestedPopupMenuTest

Similar to crrev.com/c/1298478, this patch adds a waiter to make sure
the previous RenderProcessHost get destroyed.

Tested with the trick mentioned in crrev.com/c/1299976 which could cause
consistent failures without the fix.

Bug: 889002
Change-Id: Id142ab917038e0a13eb676887525738ea08e3876
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754787Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687318}

content/browser/site_per_process_hit_test_browsertest.cc

@@ -5613,10 +5613,9 @@ IN_PROC_BROWSER_TEST_P(SitePerProcessHitTestBrowserTest,
 // On Mac and Android, the reported menu coordinates are relative to the
 // OOPIF, and its screen position is computed later, so this test isn't
 // relevant on those platforms.
-// TODO(crbug.com/889002): This test is flaky.
 #if !defined(OS_ANDROID) && !defined(OS_MACOSX)
 IN_PROC_BROWSER_TEST_P(SitePerProcessHitTestBrowserTest,
-                       DISABLED_ScrolledNestedPopupMenuTest) {
+                       ScrolledNestedPopupMenuTest) {
   GURL main_url(embedded_test_server()->GetURL(
       "a.com", "/frame_tree/page_with_tall_positioned_frame.html"));
   EXPECT_TRUE(NavigateToURL(shell(), main_url));
@@ -5630,10 +5629,18 @@ IN_PROC_BROWSER_TEST_P(SitePerProcessHitTestBrowserTest,
 
   FrameTreeNode* grandchild_node = child_node->child_at(0);
 
+  RenderProcessHost* rph = grandchild_node->current_frame_host()->GetProcess();
+  RenderProcessHostWatcher watcher(
+      rph, RenderProcessHostWatcher::WATCH_FOR_HOST_DESTRUCTION);
+
   GURL grandchild_url(embedded_test_server()->GetURL(
       "c.com", "/site_isolation/page-with-select.html"));
   NavigateFrameToURL(grandchild_node, grandchild_url);
 
+  // This is to make sure that the navigation is completed and the previous
+  // RenderProcessHost is destroyed.
+  watcher.Wait();
+
   WaitForHitTestData(grandchild_node->current_frame_host());
 
   EXPECT_EQ(