extensions/ directory contains embedder-specific, not-web-exposed APIs (e.g., not-web-exposed APIs for Chromium OS etc). The directory is useful to implement embedder-specific, not-web-exposed APIs using Blink technologies for web-exposed APIs like WebIDL, V8 bindings and Oilpan.
Remember that you should not implement any web-exposed APIs in
extensions/. Web-exposed APIs should go through the standardization process and be implemented in
modules/. Also, per the Chromium contributor guideline, code that is not used by Chromium should not be added to
In terms of dependencies,
extensions/ can depend on
platform/, but not vice versa.
extensions/ directory contains sub-directories for individual embedders (e.g.,
extensions/chromeos/). Each sub-directory is linked into the Blink link unit only when the embedder is built.