Widget opacity goes from 0 to 1, not 0 to 255.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index a1258a64d5d798a27418a5eb0c4b4697a687dd10..4aaeade757203fe7437a3619fd88e2218636e3de 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -467,6 +467,9 @@ void Layer::SetBackgroundFilters(const FilterOperations& filters) {
 
 void Layer::SetOpacity(float opacity) {
   DCHECK(IsPropertyChangeAllowed());
+  DCHECK_GE(opacity, 0.f);
+  DCHECK_LE(opacity, 1.f);
+
   if (opacity_ == opacity)
     return;
   // We need to force a property tree rebuild when opacity changes from 1 to a
@@ -1613,6 +1616,9 @@ void Layer::OnFilterAnimated(const FilterOperations& filters) {
 }
 
 void Layer::OnOpacityAnimated(float opacity) {
+  DCHECK_GE(opacity, 0.f);
+  DCHECK_LE(opacity, 1.f);
+
   if (opacity_ == opacity)
     return;
   opacity_ = opacity;