styleguide: Allow std::forward with guidance.

Uses already exist in Chromium, so this works on our platforms.

NOTRY=true

Review URL: https://codereview.chromium.org/1571483002

Cr-Commit-Position: refs/heads/master@{#368189}

Author: jeremyroman

styleguide/c++/c++11.html

@@ -394,6 +394,17 @@ <h2 id="whitelist"><a name="library-whitelist"></a>C++11 Allowed Library Feature
 <td>This is allowed, but there is almost always a way to write code without using this version of std::move. Not using it usually results in cleaner, easier to read, and less confusing code. <a href='https://groups.google.com/a/chromium.org/forum/#!topic/cxx/8WzmtYrZvQ8'>Discussion thread</a></td>
 </tr>
 
+<tr>
+<td>Forwarding references</td>
+<td><code>std::forward()</code></td>
+<td>Perfectly forwards arguments (including rvalues)</td>
+<td><a href="http://en.cppreference.com/w/cpp/utility/forward"><code>std::forward</code></a></td>
+<td>
+  Allowed, though usage should be rare (primarily for forwarding constructor arguments, or in carefully reviewed library code).
+  <a href="https://groups.google.com/a/chromium.org/d/topic/cxx/-O7euklhSxs/discussion">Discussion thread</a>
+</td>
+</tr>
+
 <tr>
 <td>Type Traits</td>
 <td>Class templates within <code>&lt;type_traits&gt;</code></td>