Disable flaky CrossSiteIframeBlockedByXFrameOptionsOrCSP on viz

BUG=790683
TBR=creis@chromium.org

Change-Id: I9ba4317841f6487795f1fb0676c9dde09f6fe500
Reviewed-on: https://chromium-review.googlesource.com/801264
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521590}

testing/buildbot/filters/mojo.fyi.viz.content_browsertests.filter

@@ -80,3 +80,6 @@
 
 # TODO: investigate flaky failure http://crbug.com/783434
 -GpuProcessHostBrowserTest.Shutdown
+
+# TODO: investigate flaky failure http://crbug.com/790683
+-SitePerProcessBrowserTest.CrossSiteIframeBlockedByXFrameOptionsOrCSP

testing/buildbot/filters/viz.content_browsertests.filter

@@ -83,3 +83,6 @@
 
 # OnGpuSwapBuffersCompletedInternal not called. http://crbug.com/791557
 -MouseLatencyBrowserTest.CoalescedMouseMovesCorrectlyTerminated
+
+# TODO: investigate flaky failure http://crbug.com/790683
+-SitePerProcessBrowserTest.CrossSiteIframeBlockedByXFrameOptionsOrCSP