base: Add/Fix namespace closing comments.

A namespace foo should end with a comment "// namespace foo", this
corrects errors in base/.

BUG=464816

Review URL: https://codereview.chromium.org/981803003

Cr-Commit-Position: refs/heads/master@{#319543}
diff --git a/base/strings/utf_offset_string_conversions_unittest.cc b/base/strings/utf_offset_string_conversions_unittest.cc
index 529939f..9398a56 100644
--- a/base/strings/utf_offset_string_conversions_unittest.cc
+++ b/base/strings/utf_offset_string_conversions_unittest.cc
@@ -293,4 +293,4 @@
   EXPECT_EQ(2u, adjustments_on_adjusted_string[5].output_length);
 }
 
-}  // namaspace base
+}  // namespace base