supersize: Don't cluster by default. Make Diff() accept only SizeInfo.

tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden

Index: tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden
diff --git a/tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden b/tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden
index 445e31092cc7c6b5b668c8a409d09b9ff5be02e5..66af3f0b7d8f546aa70a8b20ba40e410da821751 100644
--- a/tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden
+++ b/tools/binary_size/libsupersize/testdata/SymbolGroupMethods.golden
@@ -5,7 +5,7 @@ Number of object files: 5
 
 First columns are: running total, address, pss
  3245548 *@Group      3245548 {no path}
-             {global} (count=26)
+             {global} (count=28)
  4035476 *@Group      789928  {no path}
              chrome::mojom (count=2)
  4035676 *@Group      200     third_party/icu/ucnv_ext.c
@@ -31,7 +31,7 @@ Number of object files: 4
 
 First columns are: running total, address, pss
  3245548 *@Group      3245548 {no path}
-             {global} (count=26)
+             {global} (count=28)
  4035508 *@Group      789960  {no path}
              chrome (count=3)
  4035708 *@Group      200     third_party/icu/ucnv_ext.c
@@ -70,7 +70,7 @@ Number of object files: 4
 
 First columns are: running total, address, pss
  3245548 *@Group      3245548 {no path}
-             {global} (count=26)
+             {global} (count=28)
  4035508 *@Group      789960  {no path}
              chrome (count=3)
  4035662 *@Group      154     {no path}