Update element_id class comment to reflect how element id works

cc/trees/element_id.h

Index: cc/trees/element_id.h
diff --git a/cc/trees/element_id.h b/cc/trees/element_id.h
index 74de4c9995f9ea84733dcd2cf2520e3c17695c10..75d3601f690a3e744bfe6159870774ee5191cdac 100644
--- a/cc/trees/element_id.h
+++ b/cc/trees/element_id.h
@@ -24,16 +24,23 @@ class TracedValue;
 
 namespace cc {
 
-// An "element" is really an animation target. It retains the name element to be
-// symmetric with ElementAnimations and blink::ElementAnimations, but is not
-// in fact tied to the notion of a blink element. It is also not associated with
-// the notion of a Layer. Ultimately, these ids will be used to look up the
-// property tree node associated with the given animation.
+// Element ids are chosen by cc's clients and can be used as a stable identifier
+// across updates.
 //
-// These ids are chosen by cc's clients to permit the destruction and
-// restoration of cc entities (when visuals are hidden and shown) but maintain
-// stable identifiers. There will be a single layer for an ElementId, but
-// not every layer will have an id.
+// Historically, the layer tree stored all compositing data but this has been
+// refactored over time into auxilliary structures such as property trees.
+//
+// In composited scrolling, Layers directly reference scroll tree nodes
+// (Layer::scroll_tree_index) but scroll tree nodes are being refactored to
+// reference stable element ids instead of layers. Scroll property nodes have
+// unique element ids that blink creates from scrollable areas (though this is
+// opaque to the compositor). This refactoring of tracking the scrolling element
+// id instead of the scrolling layer id allows for more general compositing
+// where, for example, multiple layers scroll with a single scroll node.
+//
+// The animation system (see: ElementAnimations and blink::ElementAnimations) is
+// another auxilliary structure to the layer tree and uses element ids as a
+// stable identifier for animation targets.

[ajuma, 2017/04/18 18:53:57]

It's worth also mentioning that we currently set ElementIds on layers, and those
can change over the course of the layer's lifetime since, for animations,
non-default ElementIds are only set during the animation's lifetime.

 struct CC_EXPORT ElementId {
   ElementId(int primaryId, int secondaryId)
       : primaryId(primaryId), secondaryId(secondaryId) {}