Move lengthy inline functions from headers file to the .cc file.

This is following the C++ style guide: "Define functions inline only when they are small, say, 10 lines or fewer. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size."

No behavior change. not protected.

PiperOrigin-RevId: 325052309
Change-Id: Iede30d11e3ca118e763db000d11f56fbd7cae1f6