Currently WebKit related header files that need to be referenced both from browser and renderer are all put in WebKit/common (except for the legacy ones in WebKit/public), but for the files that are also referenced from WebKit directories they should probably be placed in WebKit/public/common.
See also the public discussion thread:
https://groups.google.com/a/chromium.org/forum/#!topic/platform-architecture-dev/Ala6ZxFsFPQ
Comment 1 by bugdroid1@chromium.org
, Feb 9 2018