Bug 862918 - Only pass aIsModalContentWindow if we're actually a content docshell. r=bz

Currently, the parent browser.xul docshell ends up getting created as an
nsGlobalModalWindow, because we pass nsIWebBrowserChrome::CHROME_OPENAS_CHROME
from nsWindowWatcher when creating the initial container window. The other
plausible thing to do here would be to add CHROME_OPENAS_CHROME to the call to
CreateTopLevelWindow in nsXULWindow::CreateNewContentWindow, since it's very
clear there that we're just creating the containing browser.xul docshell and not
creating the actual docshell of interest to the caller. But that seems more likely
to have unintended consequences, so we fix it here.

docshell/base/nsDocShell.cpp

@@ -11416,9 +11416,8 @@ nsDocShell::EnsureScriptEnvironment()
     uint32_t chromeFlags;
     browserChrome->GetChromeFlags(&chromeFlags);
 
-    bool isModalContentWindow =
-        (chromeFlags & nsIWebBrowserChrome::CHROME_MODAL) &&
-        !(chromeFlags & nsIWebBrowserChrome::CHROME_OPENAS_CHROME);
+    bool isModalContentWindow = (mItemType == typeContent) &&
+        (chromeFlags & nsIWebBrowserChrome::CHROME_MODAL);
 
     // If our window is modal and we're not opened as chrome, make
     // this window a modal content window.