Fix shutdown memory leak

[brendandahl]

[39399] WARNING: nsAppShell::Exit() called redundantly: file /Users/bdahl/projects/positron/widget/cocoa/nsAppShell.mm, line 678
[39399] WARNING: NS_ENSURE_TRUE(maybeContext) failed: file /Users/bdahl/projects/positron/xpcom/threads/nsThread.cpp, line 935
[39399] WARNING: 'NS_FAILED(RemovePermissionChangeObserver())', file /Users/bdahl/projects/positron/dom/notification/Notification.cpp, line 674
--DOCSHELL 0x125116800 == 1 [pid = 39399] [id = 1]
--DOCSHELL 0x128873800 == 0 [pid = 39399] [id = 2]
--DOMWINDOW == 3 (0x125117000) [pid = 39399] [serial = 1] [outer = 0x0] [url = resource://gre-resources/hiddenWindow.html]
--DOMWINDOW == 2 (0x128874800) [pid = 39399] [serial = 3] [outer = 0x0] [url = file:///Users/bdahl/projects/positron/positron/test/hello-world/index.html]
--DOMWINDOW == 1 (0x11b917800) [pid = 39399] [serial = 2] [outer = 0x0] [url = about:blank]
[39399] WARNING: Extra shutdown CC: 'i < NORMAL_SHUTDOWN_COLLECTIONS', file /Users/bdahl/projects/positron/xpcom/base/nsCycleCollector.cpp, line 3586
WARNING: YOU ARE LEAKING THE WORLD (at least one JSRuntime and everything alive inside it, that is) AT JS_ShutDown TIME.  FIX THIS!
[39399] WARNING: '!compMgr', file /Users/bdahl/projects/positron/xpcom/glue/nsComponentManagerUtils.cpp, line 63
Leaked URLs:
  resource://gre-resources/counterstyles.css
  chrome://global/content/minimal-xul.css
  resource://gre-resources/quirk.css
  resource://gre/res/svg.css
  chrome://global/content/xul.css
  chrome://global/skin/scrollbars.css
  resource://gre-resources/number-control.css
  resource://gre-resources/forms.css
  resource://gre-resources/noscript.css
  resource://gre-resources/html.css
  resource://gre-resources/ua.css
  chrome://pluginproblem/content/pluginFinderBinding.css
  chrome://pluginproblem/content/pluginProblemBinding.css
  chrome://global/content/bindings/scrollbar.xml#scrollbar
  chrome://global/content/bindings/scrollbar.xml#thumb
  chrome://global/content/bindings/scrollbar.xml#scrollbar-base
  file:///Users/bdahl/projects/positron/positron/chrome/content/shell.html
  file:///Users/bdahl/projects/positron/positron/test/hello-world/index.html
  file:///Users/bdahl/projects/positron/positron/test/hello-world/index.html
[39399] WARNING: XPCOM objects created/destroyed from static ctor/dtor: file /Users/bdahl/projects/positron/xpcom/base/nsTraceRefcnt.cpp, line 171
[39399] WARNING: XPCOM objects created/destroyed from static ctor/dtor: file /Users/bdahl/projects/positron/xpcom/base/nsTraceRefcnt.cpp, line 171
nsStringStats
 => mAllocCount:          23651
 => mReallocCount:          456
 => mFreeCount:           21498  --  LEAKED 2153 !!!
 => mShareCount:          16266
 => mAdoptCount:           1101
 => mAdoptFreeCount:       1099  --  LEAKED 2 !!!
 => Process ID: 39399, Thread ID: 140735256973312