Reenable the SitePerProcessBrowserTest.ScrollBubblingFromOOPIFTest test.

As explained in https://crbug.com/889301#c5, the test failure in
single_process_mash_content_browsertests on linux-chromeos-rel is
already expected and covered by an entry in
//testing/buildbot/filters/chromeos.single_process_mash.content_browsertests.filter:
    -SitePerProcessBrowserTest.*

Therefore, there is no point in additionally disabling the test via
//content/browser/site_per_process_browsertest.cc

Bug: 889301
Change-Id: Id86d39792939c06b2972443ea8caecad774ee6e1
Reviewed-on: https://chromium-review.googlesource.com/1246285Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594400}

content/browser/site_per_process_browsertest.cc

@@ -1328,9 +1328,7 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest,
 
 // Test that scrolling a nested out-of-process iframe bubbles unused scroll
 // delta to a parent frame.
-// TODO(https://crbug.com/889301): Re-enable this test.
-IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest,
-                       DISABLED_ScrollBubblingFromOOPIFTest) {
+IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest, ScrollBubblingFromOOPIFTest) {
   ui::GestureConfiguration::GetInstance()->set_scroll_debounce_interval_in_ms(
       0);
   GURL main_url(embedded_test_server()->GetURL(