Clarify requirements for third-party additions

docs/adding_to_third_party.md

 # Adding third_party Libraries
 
 [TOC]
 
 Using third party code can save time and is consistent with our values - no need
 to reinvent the wheel! We put all code that isn't written by Chromium developers
 into src/third_party (even if you end up modifying just a few functions). We do
 this to make it easy to track license compliance, security patches, and supply
 the right credit and attributions. It also makes it a lot easier for other
 projects that embed our code to track what is Chromium licensed and what is
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 untracked. That is, anyone running `git status` from `src/` would see a clutter.
 Your project's files are tracked by your repository, not Chromium's, so make
 sure the directory is listed in Chromium's `.gitignore`.
 
 ## Get a Review
 
 All third party additions and substantive changes like re-licensing need the
 following sign-offs. Some of these are accessible to Googlers only. Non-Googlers
 can email one of the people in third_party/OWNERS for help.
 
-* Chrome Eng Review. Googlers should see go/chrome-eng-review
+* Chrome Eng Review. Googlers should see go/chrome-eng-review (please include information about the additional checkout size, build times, and binary sizes. Please also make sure that the motivation for your project is clear, e.g., a design doc has been circulated).
 * open-source-third-party-reviews@google.com (ping the list with relevant
   details and a link to the CL).
 * security@chromium.org (ping the list with relevant details and a link to the
   CL).
 
+Please send separate emails to the three lists.
+
 Third party code is a hot spot for security vulnerabilities. When adding a new
 package that could potentially carry security risk, make sure to highlight risk
 to security@chromium.org. You may be asked to add a README.security or, in
 dangerous cases, README.SECURITY.URGENTLY file. When you update your code, be
 mindful of security-related mailing lists for the project and relevant CVE to
 update your package.
 
 Subsequent changes don't require third-party-owners approval; you can modify the
 code as much as you want.
 
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 * Missing license file
     * Make sure that the license file is present. It should be called 'LICENSE',
       or otherwise README.chromium file must point to it explicitly.
 * The following files contain a third-party license but are not in a listed
   third-party directory...
     * Check if it's a false positive (e.g. 'copyright' word used in a string
       literal), if so, update
       [src/tools/copyright_scanner/third_party_files_whitelist.txt](https://code.google.com/p/chromium/codesearch#chromium/src/tools/copyright_scanner/third_party_files_whitelist.txt)
       file. Otherwise, please move the code into third_party.