8274 [ui] Add null check in FontWeight.lerp (Work in progress (WIP), cla: yes, prod: API break)

8276 Make it easy to write embedder unit tests by creating a fixture and config builder. (cla: yes)

8277 Roll buildroot to 9c7b023ff266ee58b00fe60326fa1db910a087f3 (cla: yes)

8293 [vulkan] Add FUCHSIA external sem/mem extensions (cla: yes)

8295 Fixing links between higher and lower classes. (affects: docs, cla: yes)

8296 Migrate existing embedder unit tests to use the fixture. (cla: yes)

8297 [fuchsia] Add missing trace macros (cla: yes)