Allow NewStringElement inlining.

[copybara-service]

Allow NewStringElement inlining.

name                                   old INSTRUCTIONS/op     new INSTRUCTIONS/op     delta
BM_RepeatedPtrField_Ctor                  251 ± 0%                251 ± 0%     ~     (all samples are equal)
BM_RepeatedPtrField_Add_Sso              84.8 ± 2%               76.7 ± 2%   -9.51%      (p=0.000 n=100+100)
BM_RepeatedPtrField_Add_FallbackToRep     215 ± 5%                199 ± 5%   -7.26%       (p=0.000 n=100+99)
BM_RepeatedPtrField_Add_Small             163 ± 1%                157 ± 1%   -3.96%      (p=0.000 n=100+100)
BM_RepeatedPtrField_Add_Large            96.7 ± 1%               90.7 ± 1%   -6.13%      (p=0.000 n=100+100)
BM_RepeatedPtrField_Add_HasCleared       36.5 ± 0%               36.5 ± 0%     ~           (p=0.314 n=76+79)
BM_RepeatedPtrField_AddAllocated         30.5 ± 0%               30.6 ± 1%     ~           (p=0.109 n=78+93)
BM_RepeatedPtrField_Sort                49.5k ± 0%              49.5k ± 0%   +0.12%        (p=0.000 n=80+95)
BM_RepeatedPtrField_SortIndirect          559 ± 0%                561 ± 6%     ~           (p=0.203 n=76+81)