Disable BackForwardCache for WebContentsImplTest.CrossSiteNavigationBackOldNavigationIgnored test

This CL disables BackForwardCache for
WebContentsImplTest.CrossSiteNavigationBackOldNavigationIgnored
test which assumes no interaction with the back-forward cache by
performing a commit on a stored BFCached entry directly.

Bug: 1163106
Change-Id: Id21e363950e71db70e7db302b57b8e02958ebb72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633441
Commit-Queue: Sreeja Kamishetty <sreejakshetty@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846069}

content/browser/web_contents/web_contents_impl_unittest.cc

@@ -1156,6 +1156,13 @@ TEST_F(WebContentsImplTest, CrossSiteNavigationBackPreempted) {
 // Tests that if we go back twice (same-site then cross-site), and the cross-
 // site RFH commits first, we ignore the now-swapped-out RFH's commit.
 TEST_F(WebContentsImplTest, CrossSiteNavigationBackOldNavigationIgnored) {
+  // This test assumes no interaction with the back-forward cache. Indeed, it
+  // isn't possible to perform the second back navigation in between the
+  // ReadyToCommit and Commit of the first back-forward cache one. Both steps
+  // are combined with it, nothing can happen in between.
+  contents()->GetController().GetBackForwardCache().DisableForTesting(
+      BackForwardCache::TEST_ASSUMES_NO_CACHING);
+
   // Start with a web ui page, which gets a new RFH with WebUI bindings.
   GURL url1(std::string(kChromeUIScheme) + "://" +
             std::string(kChromeUIGpuHost));