Formatting cleanup for CONTRIBUTING.md (flutter/engine#26080)

Re-wrap to 80 columns and use references for style guide links.

Unofficially, this is more about triggering a build than cleaning up.

engine/src/flutter/CONTRIBUTING.md

@@ -20,25 +20,30 @@ 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.
-    For example, do not use STL collections or std::string. Exceptions:
+  - **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. For example, 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)
+- [Objective-C][objc_style] (including [Objective-C++][objcc_style])
+- [Java][java_style]
 
-C/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
 [contrib_guide]: https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md
 [engine_dev_setup]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment
+[objc_style]: https://google.github.io/styleguide/objcguide.html
+[objcc_style]: https://google.github.io/styleguide/objcguide.html#objective-c
+[java_style]: https://google.github.io/styleguide/javaguide.html
 
 
 ### Fuchsia Contributions from Googlers
 
-Googlers contributing to Fuchsia should follow the additional steps at: go/flutter-fuchsia-pr-policy.
+Googlers contributing to Fuchsia should follow the additional steps at:
+go/flutter-fuchsia-pr-policy.