PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/LayoutTests/FlagExpectations/enable-browser-side-navigation

 # These tests currently fail when run with --enable-browser-side-navigation
 # See https://crbug.com/576261
 # https://crbug.com/608372: PlzNavigate: support POST navigations
 crbug.com/608372 fast/forms/form-submit-in-submit-event.html [ Failure ]
 crbug.com/608372 fast/forms/submit-to-url-fragment.html [ Failure ]
 crbug.com/608372 fast/forms/xss-auditor-doesnt-crash-on-post-submit.html [ Timeout ]
 crbug.com/608372 fast/loader/form-state-restore-with-frames.html [ Failure ]
 crbug.com/608372 http/tests/history/back-to-post.html [ Timeout ]
 crbug.com/608372 http/tests/navigation/post-goback-same-url.html [ Failure ]
 crbug.com/608372 virtual/stable/http/tests/navigation/post-goback-same-url.html [ Failure ]
@@ skipping 103 matching lines @@
 
 # https://crbug.com/648588: Properly set origin and referrer header
 crbug.com/648588 http/tests/navigation/post-frames-goback1.html [ Failure ]
 crbug.com/648588 http/tests/navigation/post-goback1.html [ Failure ]
 crbug.com/648588 virtual/stable/http/tests/navigation/post-frames-goback1.html [ Failure ]
 crbug.com/648588 virtual/stable/http/tests/navigation/post-goback1.html [ Failure ]
 
 # Properly set ZoomLevel (see IFrameZoomBrowsertests*)
 Bug(none) fast/css/preserve-user-specified-zoom-level-on-reload.html [ Failure ]
 
-# https://crbug.com/648600: Don't output 2 blank spaces at the end of some layout tests
-crbug.com/648600 fast/overflow/overflow-height-float-not-removed-crash.html [ Failure ]
-crbug.com/648600 virtual/spv2/fast/overflow/overflow-height-float-not-removed-crash.html [ Failure ]
-
 # https://crbug.com/648608: Properly set the initator of the navigation.
 crbug.com/648608 http/tests/cookies/same-site/popup-same-site-post.html [ Failure ]
 crbug.com/648608 http/tests/cookies/same-site/popup-same-site.html [ Failure ]
 
-# https://crbug.com/657834: <object> must failback to its alternative content.
-Bug(657834) fast/overflow/overflow-height-float-not-removed-crash3.html [ Failure ]
-Bug(657834) imported/wpt/html/semantics/embedded-content/the-object-element/object-attributes.html [ Failure ]
-Bug(657834) http/tests/misc/resource-timing-sizes-tags.html [ Failure ]
-Bug(657834) fast/table/giantCellspacing.html [ Failure ]
-Bug(657834) fast/css/acid2-pixel.html [ Crash Timeout Failure ]
-Bug(657834) fast/css/acid2.html [ Crash Timeout Failure ]
-Bug(657834) virtual/spv2/fast/overflow/overflow-height-float-not-removed-crash3.html [ Failure ]
-Bug(657834) plugins/object-onfocus-mutation-crash.html [ Timeout ]
-
 # Untriaged navigation
 Bug(none) fast/dom/Document/open-with-pending-load.html [ Failure ]
 Bug(none) fast/dom/HTMLAnchorElement/anchor-no-multiple-windows.html [ Failure ]
 Bug(none) http/tests/loading/pdf-commit-load-callbacks.html [ Crash Failure ]
+Bug(none) http/tests/misc/resource-timing-sizes-tags.html [ Failure ]
 Bug(none) http/tests/navigation/forward-to-fragment-fires-onload.html [ Timeout ]
 Bug(none) http/tests/navigation/response204.html [ Timeout ]
 Bug(none) http/tests/security/cross-frame-access-parent-isolated-world.html [ Timeout ]
 Bug(none) http/tests/security/cross-frame-access-parent-explicit-domain-isolated-world.html [ Timeout ]
 Bug(none) imported/wpt/html/browsers/the-window-object/security-window/window-security.html [ Failure ]
 Bug(none) virtual/stable/http/tests/navigation/forward-to-fragment-fires-onload.html [ Timeout ]
 Bug(none) virtual/stable/http/tests/navigation/response204.html [ Timeout ]
 
 # Untriaged other
 Bug(none) http/tests/misc/policy-delegate-called-twice.html [ Failure ]
 Bug(none) http/tests/misc/window-open-then-write.html [ Timeout ]