Bug 1510569 - Keep track of whether we are navigating to a new URI in nsDocShell r=mconley,kmag,qdot

Previously the `WebNavigationChild` would keep track of when triggering its
`nsIWebNavigation`, `goForward`, `goBack`, `gotoIndex`, and `loadURI` methods.
It's `nsIWebNavigation` instance is always an `nsIDocShell` and as part of
porting `OnStateChange` and `OnLocationChange` events from
`WebProgressChild`/`RemoteWebProgress` to `BrowserChild`/`BrowserParent`, this
informations needs to be available from the `BrowserChild`. As it stands, it is
currently an expando property on the `WebProgressChild`.

Instead of introducing yet another XPCOM interface for the WebProgressChild, we
now store this information directly on the `nsDocShell`. Furthermore, instead
of having the `WebNavigationChild` manage this part of the `nsDocShell`'s
state, we can have the `nsDocShell` manage this state itself so it is always
consistent.

Differential Revision: https://phabricator.services.mozilla.com/D28124

docshell/base/nsDocShell.h

@@ -1205,6 +1205,10 @@ class nsDocShell final : public nsDocLoader,
   bool mTitleValidForCurrentURI : 1;
 
   bool mIsFrame : 1;
+
+  // This flag indicates whether or not the DocShell is currently executing an
+  // nsIWebNavigation navigation method.
+  bool mIsNavigating : 1;
 };
 
 #endif /* nsDocShell_h__ */