Add information on the requirements for a new Chromium port.

docs/new_port_policy.md

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+# Policy for Adding a New Port
+
+Since every new port for Chromium has a maintenance cost, here are some guidelines for when the project will accept a new port.
+
+## Expectations
+
+*   Ports should represent a significant ongoing investment to established platforms, rather than hobby or experimental code.
+*   These will not have bots on Google-run waterfalls (even FYI).
+*   Chromium engineers are not expected to maintain them.
+*   As much as possible, try to use existing branches/ifdefs.
+*   While changes in src/base are unavoidable, higher level directories shouldn’t have to change. i.e. existing porting APIs should be used. We would not accept new rendering pipelines as an example.
+*   Send an email to [src/OWNERS](https://chromium.googlesource.com/chromium/src/+/master/ENG_REVIEW_OWNERS) for approval.