aura: apply brightness filter last to improve perf

https://bugs.webkit.org/show_bug.cgi?id=92059 adds an optimization on color
matrix filters so that they can be combined in a single pass as long as the
resulting colors don't need to be clamped. Brightness needs clamping, so it
should go last.

This also fixes an uninitialized value that made us stick a grayscale filter randomly on layers.

BUG=138645
TEST=new startup animation


Review URL: https://chromiumcodereview.appspot.com/10833027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148837 0039d316-1c4b-4281-b951-d872f2087c98

ui/compositor/layer.cc

@@ -78,6 +78,7 @@ Layer::Layer(LayerType type)
       background_blur_radius_(0),
       layer_saturation_(0.0f),
       layer_brightness_(0.0f),
+      layer_grayscale_(0.0f),
       layer_inverted_(false),
       layer_mask_(NULL),
       layer_mask_back_link_(NULL),
@@ -288,16 +289,19 @@ void Layer::SetLayerFilters() {
     filters.append(WebKit::WebFilterOperation::createSaturateFilter(
         layer_saturation_));
   }
-  if (layer_brightness_) {
-    filters.append(WebKit::WebFilterOperation::createBrightnessFilter(
-        layer_brightness_));
-  }
   if (layer_grayscale_) {
     filters.append(WebKit::WebFilterOperation::createGrayscaleFilter(
         layer_grayscale_));
   }
   if (layer_inverted_)
     filters.append(WebKit::WebFilterOperation::createInvertFilter(1.0));
+  // Brightness goes last, because the resulting colors neeed clamping, which
+  // cause further color matrix filters to be applied separately. In this order,
+  // they all can be combined in a single pass.
+  if (layer_brightness_) {
+    filters.append(WebKit::WebFilterOperation::createBrightnessFilter(
+        layer_brightness_));
+  }
 
   web_layer_.setFilters(filters);
 }