Added comments clarifying some details of AXNode::IsEmptyLeaf and IsLeaf

TBR=aleventhal@chromium.org

Change-Id: I5d188d9751d9436992053981e99c0054bd44f89f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644892Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846438}

ui/accessibility/ax_node.cc

@@ -1286,13 +1286,15 @@ bool AXNode::IsEmptyLeaf() const {
 }
 
 bool AXNode::IsLeaf() const {
-  // A node is a leaf if it has no descendants, regardless whether it is ignored
-  // or not.
+  // A node is a leaf if it has no descendants, i.e. if it is at the bottom of
+  // the tree, regardless whether it is ignored or not.
   if (children().empty())
     return true;
 
-  // Leaf nodes with descendants should always be exposed to the platforms'
-  // accessibility layer.
+  // Ignored nodes with any kind of descendants, (ignored or unignored), cannot
+  // be leaves because: A) If some of their descendants are unignored then those
+  // descendants need to be exposed to the platform layer, and B) If all of
+  // their descendants are ignored they are still not at the bottom of the tree.
   if (IsIgnored())
     return false;
 

ui/accessibility/ax_node.h

@@ -486,6 +486,12 @@ class AX_EXPORT AXNode final {
   // that, ignored nodes (leaf or otherwise) do not expose their inner text or
   // hypertext to the platforms' accessibility layer, but they expose the inner
   // text or hypertext of their unignored descendants.
+  //
+  // For example, empty text fields might have a set of unignored nested divs
+  // inside them:
+  // ++kTextField
+  // ++++kGenericContainer
+  // ++++++kGenericContainer
   bool IsEmptyLeaf() const;
 
   // Returns true if this is a leaf node, meaning all its