Android: Add explanation for apkanalyzer.py

This is a bug that is already fixed in upstream apkanalyzer but not yet
released and rolled in the various downstream projects and ultimately
into Android SDK Tools (eventually).

Bug: 874994
Change-Id: I9c83b6ee35c5691f687e174b3213ca7e71a9ea2c
Reviewed-on: https://chromium-review.googlesource.com/1178687
Commit-Queue: agrieve <agrieve@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584049}

tools/binary_size/libsupersize/apkanalyzer.py

@@ -111,11 +111,18 @@ def UndoHierarchicalSizing(data):
       else:
         # Sibling or higher nodes
         break
-    # Disabled for perf bots: https://crbug.com/874956
-    # TODO(wnwen): Investigate reason
+
+    # Apkanalyzer may overcount private method sizes at times. Unfortunately
+    # the fix is not in the version we have in Android SDK Tools. For now we
+    # prefer to undercount child sizes since the parent's size is more
+    # accurate. This means the sum of child nodes may exceed its immediate
+    # parent node's size.
+    total_child_size = min(size, total_child_size)
+    # TODO(wnwen): Add assert back once dexlib2 2.2.5 is released and rolled.
     #assert total_child_size <= size, (
     #    'Child node total size exceeded parent node total size')
-    node_size = max(0, size - total_child_size)
+
+    node_size = size - total_child_size
     nodes.append((name, node_size))
     return next_idx, size