Add an include guard around typedef in dwrite_font_proxy_messages.h to fix ipc_fuzzer build.
Add an include guard around typedef in dwrite_font_proxy_messages.h to fix ipc_fuzzer build.

BUG=569964

Review URL: https://codereview.chromium.org/1531563003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365427}

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