chromeos: removes NOTREACHED in client_image_registry

Having a NOTREACHED here is problematic for two reasons:
. A bad client could trigger the NOTREACHED by setting a random value.
. Because the registry is a separate pipe then that to the WindowService it
  is entirely possible for the property to be set on the window before the
  property has been set in the registry.

BUG=863520
TEST=none
TBR=estade@chromium.org

Change-Id: I08b047e871c39d46cfcd548b078df6af9e334032
Reviewed-on: https://chromium-review.googlesource.com/1136937Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575045}

ash/client_image_registry.cc

@@ -19,7 +19,8 @@ const gfx::ImageSkia* ClientImageRegistry::GetImage(
     const base::UnguessableToken& token) const {
   auto iter = images_.find(token);
   if (iter == images_.end()) {
-    NOTREACHED();
+    // No DCHECK here as otherwise ash would crash if a bad client supplies a
+    // random value.
     return nullptr;
   }