Handling disappear or new sections in after.size file

tools/binary_size/libsupersize/diff.py

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Logic for diffing two SizeInfo objects."""
 
 import collections
 import re
 
 import models
 
@@ skipping 164 matching lines @@
       similar.append(models.Symbol(
           section_name, padding,
           name="** aggregate padding of diff'ed symbols"))
   return models.SymbolDiff(added, removed, similar)
 
 
 def Diff(before, after):
   """Diffs two SizeInfo objects. Returns a SizeInfoDiff."""
   assert isinstance(before, models.SizeInfo)
   assert isinstance(after, models.SizeInfo)
-  section_sizes = {k: after.section_sizes[k] - v
+  section_sizes = {k: after.section_sizes.get(k, 0) - v
                    for k, v in before.section_sizes.iteritems()}
+  for k, v in after.section_sizes.iteritems():
+    if k not in section_sizes:
+      section_sizes[k] = v
+
   symbol_diff = _DiffSymbolGroups(before.raw_symbols, after.raw_symbols)
   return models.SizeInfoDiff(section_sizes, symbol_diff, before.metadata,
                              after.metadata)