Add gn args to metadata and metadata diffing

tools/binary_size/testdata/ActualDiff.golden

Index: tools/binary_size/testdata/ActualDiff.golden
diff --git a/tools/binary_size/testdata/ActualDiff.golden b/tools/binary_size/testdata/ActualDiff.golden
index d46c125c72e2c9a57f42431177bd17919ca040c3..d7fe08ada74d31a4bafcd6d0287bd41f4e7f5870 100644
--- a/tools/binary_size/testdata/ActualDiff.golden
+++ b/tools/binary_size/testdata/ActualDiff.golden
@@ -1,46 +1,53 @@
-Metadata: 
+Common Metadata:
+    baz=yes
+    foo=1
+Old Metadata:
+    bar=[1, 3]
+New Metadata:
+    bar=[1, 2, 3]
+
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 1 symbols added (+), 1 changed (~), 1 removed (-), 42 unchanged (=)
 Showing 45 symbols with total size: 0 bytes