Mark external/wpt/resource-timing/resource_initiator_types.html as flaky

https://github.com/w3c/web-platform-tests/pull/9986 has introduced some
flakiness to the test that is currently blocking WPT imports.

TBR=lukebjerring

Bug: 822757
Change-Id: Iddc3adf65989506b72fba003a2bbbd8d9cbf062f
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/966604Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#543724}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1540,6 +1540,8 @@ crbug.com/763830 http/tests/security/cors-rfc1918/ [ Skip ]
 # were matched on pass/fail but not exact error messages.
 crbug.com/626703 external/wpt/resource-timing/resource_TAO_space.htm [ Pass Failure ]
 
+crbug.com/822757 external/wpt/resource-timing/resource_initiator_types.html [ Pass Failure ]
+
 # Working on getting the CSP tests going:
 crbug.com/694525 external/wpt/content-security-policy/object-src [ Skip ]
 crbug.com/694525 external/wpt/content-security-policy/connect-src/connect-src-eventsource-blocked.sub.html [ Skip ]