Bug 1297549: Part 3 - Modify Windows AccessibleWrap to get hwnd from TabChild under e10s; r=tbsaunde

MozReview-Commit-ID: Jn2f9EAtT8O

--HG--
extra : rebase_source : 6ee62ec6b18d22f76ce6f2b60de9b0f408a5ec92

accessible/windows/msaa/AccessibleWrap.cpp

@@ -9,6 +9,7 @@
 
 #include "Compatibility.h"
 #include "DocAccessible-inl.h"
+#include "mozilla/dom/TabChild.h"
 #include "mozilla/a11y/DocAccessibleChild.h"
 #include "mozilla/a11y/DocAccessibleParent.h"
 #include "EnumVariant.h"
@@ -1295,6 +1296,22 @@ AccessibleWrap::GetHWNDFor(Accessible* aAccessible)
     return nullptr;
   }
 
+  if (XRE_IsContentProcess()) {
+    DocAccessible* doc = aAccessible->Document();
+    if (!doc) {
+      return nullptr;
+    }
+
+    DocAccessibleChild* ipcDoc = doc->IPCDoc();
+    if (!ipcDoc) {
+      return nullptr;
+    }
+
+    auto tab = static_cast<dom::TabChild*>(ipcDoc->Manager());
+    MOZ_ASSERT(tab);
+    return reinterpret_cast<HWND>(tab->GetNativeWindowHandle());
+  }
+
   // Accessibles in child processes are said to have the HWND of the window
   // their tab is within.  Popups are always in the parent process, and so
   // never proxied, which means this is basically correct.