Mojo: Make GetDefault{AsyncWaiter,Logger} static methods of Environment.

mojo/public/cpp/DEPS

 include_rules = [
   # Require explicit dependencies in each directory.
   "-mojo/public",
   # But everyone can depend on the C and C++ system headers.
   "+mojo/public/c/system",
   "+mojo/public/cpp/system",
+  # Ditto for the C environment headers (but not the C++ environment, since it
+  # has dependencies of its own).
+  "+mojo/public/c/environment",
 ]
 
 specific_include_rules = {
   r".*_(unit|perf)test\.cc": [
     "+testing",
     "+mojo/public/cpp/test_support",
     "+mojo/public/cpp/utility",
   ],
 }