This is to prevent a collision with Expectations class defined in test-field-type-tracking.cc, which happens in Clang under certain conditions (official build + Precise + unknown).

While the original intent was to just workaround the Clang bug, putting types into anonymous namespace seems reasonable anyway (thank you to Benedikt Meurer for the suggestion!)

BUG=630335

Review-Url: https://codereview.chromium.org/2227073002
Cr-Commit-Position: refs/heads/master@{#38552}