Add a style note about Linux embedding style (flutter#21819)

CONTRIBUTING.md

@@ -20,11 +20,18 @@ contributing guide.
 
 The Flutter engine follows Google style for the languages it uses:
 - [C++](https://google.github.io/styleguide/cppguide.html)
+  - **Note**: The Linux embedding generally follows idiomatic GObject-based C style.
+    Use of C++ is discouraged in that embedding to avoid creating hybrid code that
+    feels unfamiliar to either developers used to working with GObject or C++ developers.
+    E.g., do not use STL collections or std::string. Exceptions:
+      - C-style casts are forbidden; use C++ casts.
+      - Use `nullptr` rather than `NULL`.
+      - Avoid `#define`; for internal constants use `static constexpr` instead.
 - [Objective-C](https://google.github.io/styleguide/objcguide.html) (including
   [Objective-C++](https://google.github.io/styleguide/objcguide.html#objective-c))
 - [Java](https://google.github.io/styleguide/javaguide.html)
 
-C++ and Objective-C/C++ files are formatted with `clang-format`, and GN files with `gn format`.
+C/C++ and Objective-C/C++ files are formatted with `clang-format`, and GN files with `gn format`.
 
 [build_status]: https://cirrus-ci.com/github/flutter/engine
 [code_of_conduct]: https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md