[bfcache] Add a method that can disable the BackForwardCache in tests.

This method allows tests to force not using the BackForwardCache, this
is useful when:
  * Tests rely on a new document being loaded.
  * Tests want to specifically test the document not being in the cache.
  * Any other reason BFCache doesn't make sense for a particular test.

Change-Id: I07fbc0a31023a78109268c100e5f454dbfbed13f
Bug: 973855,965925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657836Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683552}

content/browser/back_forward_cache_browsertest.cc

@@ -809,6 +809,25 @@ IN_PROC_BROWSER_TEST_F(BackForwardCacheBrowserTest, DoesNotCacheIfWebGL) {
 
   // The page had an active WebGL context when we navigated away,
   // so it shouldn't have been cached.
+}
+
+IN_PROC_BROWSER_TEST_F(BackForwardCacheBrowserTest,
+                       DisableBackforwardCacheForTesting) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  // Disable the BackForwardCache.
+  web_contents()->GetController().back_forward_cache().DisableForTesting();
+
+  // Navigate to a page that would normally be cacheable.
+  NavigateToURL(shell(),
+                embedded_test_server()->GetURL("a.com", "/title1.html"));
+  RenderFrameDeletedObserver delete_rfh_a(current_frame_host());
+
+  // Navigate away.
+  NavigateToURL(shell(),
+                embedded_test_server()->GetURL("b.com", "/title1.html"));
+
+  // The page should be deleted (not cached).
   delete_rfh_a.WaitUntilDeleted();
 }
 

content/browser/frame_host/back_forward_cache.cc

@@ -93,7 +93,7 @@ bool BackForwardCache::CanStoreDocument(RenderFrameHostImpl* rfh) {
   if (rfh->GetParent())
     return false;
 
-  if (!IsBackForwardCacheEnabled())
+  if (!IsBackForwardCacheEnabled() || is_disabled_for_testing_)
     return false;
 
   // Note that we check is_loading on the rfh directly, rather than calling
@@ -174,9 +174,17 @@ std::unique_ptr<RenderFrameHostImpl> BackForwardCache::RestoreDocument(
   return rfh;
 }
 
-// Remove all entries from the BackForwardCache.
 void BackForwardCache::Flush() {
   render_frame_hosts_.clear();
 }
 
+void BackForwardCache::DisableForTesting() {
+  is_disabled_for_testing_ = true;
+
+  // This could happen if a test populated some pages in the cache, then
+  // called DisableForTesting(). This is not something we currently expect tests
+  // to do.
+  DCHECK(render_frame_hosts_.empty());
+}
+
 }  // namespace content

content/browser/frame_host/back_forward_cache.h

@@ -49,6 +49,17 @@ class CONTENT_EXPORT BackForwardCache {
   // Remove all entries from the BackForwardCache.
   void Flush();
 
+  // Disables the BackForwardCache so that no documents will be stored/served.
+  // This allows tests to "force" not using the BackForwardCache, this can be
+  // useful when:
+  // * Tests rely on a new document being loaded.
+  // * Tests want to test this case specifically.
+  // Note: It's preferable to make tests BackForwardCache compatible
+  // when feasible, rather than using this method. Also please consider whether
+  // you actually should have 2 tests, one with the document cached
+  // (BackForwardCache enabled), and one without.
+  void DisableForTesting();
+
  private:
   // Contains the set of stored RenderFrameHost.
   // Invariant:
@@ -56,6 +67,9 @@ class CONTENT_EXPORT BackForwardCache {
   // - Once the list is full, the least recently used document is evicted.
   std::list<std::unique_ptr<RenderFrameHostImpl>> render_frame_hosts_;
 
+  // Whether the BackforwardCached has been disabled for testing.
+  bool is_disabled_for_testing_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(BackForwardCache);
 };
 }  // namespace content