Fuchsia platform versioning

When building for Fuchsia, the binaries are built against a target API level, which is set via fuchsia_target_api_level in the .gn file. This information becomes embedded in the compiled component as a target ABI revision, which indicates the semantics the component expects from the platform.

Updates to the fuchsia_target_api_level are currently done manually. As a general rule, we want to be using the latest API level supported by the Fuchsia SDK checked out. An older API level might be needed if one of the following are true:

1. There are unresolved compatibility issues.
2. The binaries are intended to ship on an older release of Fuchsia.

For instance, if M97 is shipped on a Fuchsia release that only supports API level up to 5, the target API level cannot be updated in Chromium until M97 branch is cut.