If the mapping callback is not set or it the callback returns invalid data, ICU initialization will be embedder responsibility.
This affects all embedders and the following have been audited:
* Android: Via a symbol mapping.
* iOS: Via a file mapping.
* Embedder: Via a file mapping.
* Fuchsia: Via a VMO mapping
* Test shells and Flutter tester: Via file mapping with ICU data needing to be next to the executable.
Also adds //flutter/benchmarking which, similar to //flutter/testing, allows for the creation of a benchmarking executable. This is also the target that contains benchmarking utilities.
