Filter SitePerProcessBrowserTest.SlowUnloadHandlerInIframe.

It was thought to be fixed by https://crrev.com/c/1883352 but has been
failing on the bots since that CL (which removed the filter).

https://ci.chromium.org/p/chromium/builders/ci/linux-bfcache-debug/846
https://ci.chromium.org/p/chromium/builders/ci/android-bfcache-debug/1337

TBR=rakina@chromium.org

Change-Id: I297603be1ad8804a93ef6ec4aeccc85fe1edd766
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892975Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711727}

testing/buildbot/filters/bfcache.content_browsertests.filter

@@ -20,6 +20,7 @@
 # Test expects the RenderFrameHost to be deleted after a navigation. It is now
 # stored into the BackForwardCache instead. It isn't deleted.
 -RenderFrameHostManagerTest.CleanupOnCrossProcessNavigation
+-SitePerProcessBrowserTest.SlowUnloadHandlerInIframe
 -WebContentsBindingSetBrowserTest.CloseOnFrameDeletion
 
 # Wait for the old process to exit. It won't, because it is still used by a