styleguide: Make default function template arguments an allowed feature.
styleguide: Make default function template arguments an allowed feature.

This patch moves default function template arguments from being
"to be discussed" to being allowed.

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1515493003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364132}

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