Enable InterstitialPageImplTest.UnderlyingSubframeCommit

IsBrowserSideNavigationEnabled() is now always returning true, causing
this test to be disabled. It does pass now, so we can potentially
enable it if it is still valid.

Bug: 789577
Change-Id: I85438a626b6b764c4ad07e6e74d7cb08c8adad4a
Reviewed-on: https://chromium-review.googlesource.com/c/1394932Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619810}

content/browser/frame_host/interstitial_page_impl_browsertest.cc

@@ -318,12 +318,6 @@ IN_PROC_BROWSER_TEST_F(InterstitialPageImplTest, FocusAfterDetaching) {
 // commits in the original page while an interstitial is showing.
 // See https://crbug.com/729105.
 IN_PROC_BROWSER_TEST_F(InterstitialPageImplTest, UnderlyingSubframeCommit) {
-  // This test doesn't apply in PlzNavigate, since the subframe does not
-  // succesfully commit in that mode.
-  // TODO(creis, clamy): Determine if this is a bug that should be fixed.
-  if (IsBrowserSideNavigationEnabled())
-    return;
-
   ASSERT_TRUE(embedded_test_server()->Start());
 
   // Load an initial page and inject an iframe that won't commit yet.