Deprecate goog.structs.forEach.
Deprecate goog.structs.forEach.

This will provide a slightly safer canary before deprecating goog.structs.Map and goog.structs.Set.

RELNOTES: n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154103079

teppeis/closure-library,google/closure-library,teppeis/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,ikabirov/closure-library,google/closure-library,google/closure-library,platinumpixs/google-closure-library,google/closure-library,lucidsoftware/closure-library,ikabirov/closure-library,ikabirov/closure-library,teppeis/closure-library,platinumpixs/google-closure-library,platinumpixs/google-closure-library,ikabirov/closure-library,google/closure-library,lucidsoftware/closure-library