Bug 1735856 - Don't trigger violation events for frame-ancestors r=freddyb

Differential Revision: https://phabricator.services.mozilla.com/D134145

dom/security/nsCSPContext.cpp

@@ -1442,8 +1442,12 @@ class CSPReportSenderRunnable final : public Runnable {
     }
 
     // 4) fire violation event
-    mCSPContext->FireViolationEvent(mTriggeringElement, mCSPEventListener,
-                                    init);
+    // A frame-ancestors violation has occurred, but we should not dispatch the
+    // violation event to a potentially cross-origin ancestor.
+    if (!mViolatedDirective.EqualsLiteral("frame-ancestors")) {
+      mCSPContext->FireViolationEvent(mTriggeringElement, mCSPEventListener,
+                                      init);
+    }
 
     return NS_OK;
   }

testing/web-platform/meta/content-security-policy/frame-ancestors/frame-ancestors-none-block.html.ini

@@ -1,4 +0,0 @@
-[frame-ancestors-none-block.html]
-  [frame-ancestors-none-block]
-    expected: FAIL
-