Bug 1438697 - Add inFrameSwap property on page transition. r=mrbkap

During a frame swap (using the `swapFrameLoaders` API on a frame loader owner),
we dispatch `pagehide` events before swapping and `pageshow` events after
swapping.

For some consumers that watch these events (such as DevTools), it would be
helpful to filter out the ones that sent because of frame swapping.  This change
adds a chrome-only `inFrameSwap` property to such events so that we can tell
them apart.

MozReview-Commit-ID: QzIDuaF2zC

--HG--
extra : rebase_source : 3ac15e34c18b648c4cb060e69587e39e4ebaab60

dom/base/nsDocument.cpp

@@ -8451,6 +8451,9 @@ nsDocument::DispatchPageTransition(EventTarget* aDispatchTarget,
   init.mCancelable = true;
   init.mPersisted = aPersisted;
 
+  nsDocShell* docShell = mDocumentContainer.get();
+  init.mInFrameSwap = docShell && docShell->InFrameSwap();
+
   RefPtr<PageTransitionEvent> event =
     PageTransitionEvent::Constructor(this, aType, init);
 

dom/base/test/chrome/window_swapFrameLoaders.xul

@@ -121,6 +121,9 @@ Test swapFrameLoaders with different frame types and remoteness
               running: exists && !this.api._shuttingDown,
             });
           });
+          addEventListener("pagehide", function({ inFrameSwap }) {
+            sendAsyncMessage("pagehide", inFrameSwap);
+          });
         }`;
       }
 
@@ -169,9 +172,17 @@ Test swapFrameLoaders with different frame types and remoteness
         let mmA = frameA.frameLoader.messageManager;
         let mmB = frameB.frameLoader.messageManager;
 
+        let pagehideA = once(mmA, "pagehide");
+        let pagehideB = once(mmB, "pagehide");
+
         info("swapFrameLoaders");
         frameA.swapFrameLoaders(frameB);
 
+        let [ { data: inFrameSwapA } ] = await pagehideA;
+        ok(inFrameSwapA, "Frame A got pagehide with inFrameSwap: true");
+        let [ { data: inFrameSwapB } ] = await pagehideB;
+        ok(inFrameSwapB, "Frame B got pagehide with inFrameSwap: true");
+
         let inflightA = once(mmA, "pong");
         let inflightB = once(mmB, "pong");
 

dom/webidl/PageTransitionEvent.webidl

@@ -19,9 +19,14 @@ interface PageTransitionEvent : Event
    * session history, |persisted| is true for the PageHide event.
    */
   readonly attribute boolean persisted;
+
+  // Whether the document is in the middle of a frame swap.
+  [ChromeOnly]
+  readonly attribute boolean inFrameSwap;
 };
 
 dictionary PageTransitionEventInit : EventInit
 {
   boolean persisted = false;
+  boolean inFrameSwap = false;
 };