[Pivot Table] Add descendant_slice_by_stack dynamic table

Added enum types to descendant generator and added
descendant_slice_by_stack operator that takes a stack_id and finds all
slice ids that have that stack_id, then finds all descendants of all the
slice ids similarly to descendant_slice.
Next Step: Add stack expansion to pivot table.

Bug: 149193812
Change-Id: Id8ae9b0b34eb2f82825fca161b3f3c20e4e00844
diff --git a/CHANGELOG b/CHANGELOG
index e9b0ab1..e8e0dd2 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -3,8 +3,8 @@
     * Removed DCHECK that would cause crashes when a debug build of the service
       is used with a producer built with -DNDEBUG.
   Trace Processor:
-    * Added "ancestor_slice_by_stack" table function to lookup slice ancestors
-      based on their stack_id.
+    * Added "ancestor_slice_by_stack" and "descendant_slice_by_stack" table
+      functions to walk up and down the slice stacks.
   UI:
     *
   SDK: