Refactoring RenderLayerCompositor to avoid redundant checks for Compositing reasons

Source/core/rendering/RenderLayerCompositor.cpp

Index: Source/core/rendering/RenderLayerCompositor.cpp
diff --git a/Source/core/rendering/RenderLayerCompositor.cpp b/Source/core/rendering/RenderLayerCompositor.cpp
index f472d127bcb8eb2dfaba789a65cf4fab397f1700..ff2a71e54fef71e39cd825d0dca290106f3a942d 100644
--- a/Source/core/rendering/RenderLayerCompositor.cpp
+++ b/Source/core/rendering/RenderLayerCompositor.cpp
@@ -1576,23 +1576,17 @@ CompositingReasons RenderLayerCompositor::directReasonsForCompositing(const Rend
 
     if (requiresCompositingForVideo(renderer))
         directReasons |= CompositingReasonVideo;
-
-    if (requiresCompositingForCanvas(renderer))
+    else if (requiresCompositingForCanvas(renderer))

[shawnsingh, 2013/07/01 06:23:28]

Let's add a brief comment at the beginning of this group of if-else statements,
saying "Only zero or one of the following conditions will be true for a given
RenderLayer."

I'm curious - is this just a cleanup refactoring you thought would be
appropriate?  Or is there a deeper reason that we need to use else-statements
here?  Can you please explain?

[pals, 2013/07/01 07:41:57]

Done.
This is just cleanup refactoring which I came accross while going through the
code.

         directReasons |= CompositingReasonCanvas;
-
-    if (requiresCompositingForPlugin(renderer))
+    else if (requiresCompositingForPlugin(renderer))
         directReasons |= CompositingReasonPlugin;
-
-    if (requiresCompositingForFrame(renderer))
+    else if (requiresCompositingForFrame(renderer))
         directReasons |= CompositingReasonIFrame;
     
     if (requiresCompositingForBackfaceVisibilityHidden(renderer))
         directReasons |= CompositingReasonBackfaceVisibilityHidden;
 
-    if (requiresCompositingForAnimation(renderer))
-        directReasons |= CompositingReasonAnimation;
-
-    if (requiresCompositingForTransition(renderer))
+    if (requiresCompositingForTransition(renderer) || requiresCompositingForAnimation(renderer))

[shawnsingh, 2013/07/01 06:23:28]

This particular line, let's not combine these.  We only combined them
temporarily while we were limited to 32 reasons in a 32-bit field.  We're
migrating to 64-bit already, and we should be able to add the
CompositingReasonTransition reason again.

[pals, 2013/07/01 07:41:57]

Done.

         directReasons |= CompositingReasonAnimation;
 
     if (requiresCompositingForFilters(renderer))