[SuperSize] Round padding values in .sizediff.

When creating symbol diffs, mismatches in alias counts and summing
fractional padding PSS values can lead to DiffSymbols with non-integer
padding values. However, numeric values in .sizediff files should be
integers. This CL implements a simple fix: Round paddings to integer
when creating DiffSymbols. The resulting inaccuracy would be in the
order of bytes, and therefore negligible.

Bug: 1132394
Change-Id: Iba9acb0a94de6890aec01ac6c9ee450fb9d69dd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2432668
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810827}

tools/binary_size/libsupersize/diff.py

@@ -115,6 +115,8 @@ def _DiffSymbolGroups(containers, before, after):
 
   # Create a DeltaSymbol to represent the zero'd out padding of matched symbols.
   for (container_name, section_name), padding in padding_by_segment.items():
+    # Values need to be integer (crbug.com/1132394).
+    padding = round(padding)
     if padding != 0:
       after_sym = models.Symbol(section_name, padding)
       after_sym.container = container_from_name[container_name]