src: fix unused private field warning

This fixes the following warning (and subsequent warnings that occurred from fixing the original warning):

../src/inspector/runtime_agent.h:26:16: warning: private field 'env_' is not used [-Wunused-private-field]
  Environment* env_;
               ^
1 warning generated.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines