ui: Add a DCHECK that we don't set up ContextFactory more than once.

Doing this is weird behaviour and could cause a leak, we should only
set up a ContextFactory once, and clear it before we set a new one.

[email protected]
BUG=270918

Review URL: https://codereview.chromium.org/173503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252591 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index d3347745..9130273 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -80,6 +80,7 @@
 
 // static
 void ContextFactory::SetInstance(ContextFactory* instance) {
+  DCHECK_NE(!!g_context_factory, !!instance);
   g_context_factory = instance;
 }