FR: FirebaseInstanceID macOS support

[gregbzh]

[REQUIRED] Step 2: Describe your environment

• Xcode version: 9.0.1
• Firebase SDK version: n/a
• Firebase Component: Messaging
• Component version:

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

I am currently porting Messaging on macOS. Unfortunately the FirebaseInstanceID has not been compiled for macOS. Since FirebaseInstanceID is not open source, it could be nice to add macOS support to FirebaseInstanceID.

[paulb777]

Should be easily feasible after #186

[morganchen12]

Fixed in #2880.