Revert of Re-enable layer squashing (https://codereview.chromium.org/267343005/)

Reason for revert:
We still have open layer squashing bugs.

Original issue's description:
> Re-enable layer squashing
> 
> We've fixed all the know bugs. Hopefully this time it will stick. If you find
> that this CL caused a regression, please mark the bug as blocking
> http://crbug.com/365701
> 
> BUG=365701
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173696

TBR=chrishtr@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=365701

Review URL: https://codereview.chromium.org/279703002

git-svn-id: svn://svn.chromium.org/blink/trunk@173721 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -69,7 +69,7 @@ ImageDataConstructor status=experimental
 IndexedDBExperimental status=experimental
 InputModeAttribute status=experimental
 LangAttributeAwareFormControlUI
-LayerSquashing status=stable
+LayerSquashing status=test
 PrefixedEncryptedMedia status=stable
 LocalStorage status=stable
 Manifest status=test