PlzNavigate: update layout test filter

This CL re-adds some layout tests to the layout test filter for
PlzNavigate. These tests pass locally but are failing on the bot.
Marking this as NOTRY since this is only affecting the FYI bot
LinuxBrowserSideNavigation.

BUG=576261
NOTRY=TRUE
TBR=nasko@chromium.org

Review-Url: https://codereview.chromium.org/2573843002
Cr-Commit-Position: refs/heads/master@{#438210}

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

@@ -183,3 +183,8 @@ Bug(none) imported/wpt/html/browsers/origin/cross-origin-objects/cross-origin-ob
 # https://crbug.com/673751 Policy delegate is called twice
 crbug.com/673751 http/tests/misc/policy-delegate-called-twice.html [ Failure ]
 crbug.com/673751 virtual/mojo-loading/http/tests/misc/policy-delegate-called-twice.html [ Failure ]
+
+# These tests are flaky.
+Bug(none) http/tests/misc/window-open-then-write.html [ Timeout ]
+Bug(none) imported/wpt/html/browsers/browsing-the-web/unloading-documents/beforeunload-on-history-back.html [ Failure ]
+Bug(none) virtual/mojo-loading/http/tests/misc/window-open-then-write.html [ Timeout ]