Add using statement for tensorflow::string.
Add using statement for tensorflow::string.

This fixes compile errors like the following:

ERROR: /tmpfs/tensor_flow/tensorflow/c/eager/BUILD:172:1: Couldn't build file tensorflow/c/eager/_objs/c_api_remote_test_gpu/c_api_remote_test.o: C++ compilation of rule '//tensorflow/c/eager:c_api_remote_test_gpu' failed (Exit 1)
tensorflow/c/eager/c_api_remote_test.cc:28:42: error: 'string' does not name a type; did you mean 'stdin'?
 tensorflow::ServerDef GetServerDef(const string& job_name, int num_tasks) {
                                          ^~~~~~
                                          stdin
PiperOrigin-RevId: 275565532
Change-Id: I7783fe133e06a574e00a8d9a97c00ffef493bf92

aam-at/tensorflow,sarvex/tensorflow,Intel-Corporation/tensorflow,aam-at/tensorflow,paolodedios/tensorflow,frreiss/tensorflow-fred,gunan/tensorflow,karllessard/tensorflow,jhseu/tensorflow,xzturn/tensorflow,arborh/tensorflow,paolodedios/tensorflow,ppwwyyxx/tensorflow,petewarden/tensorflow,xzturn/tensorflow,aldian/tensorflow,renyi533/tensorflow,gautam1858/tensorflow,petewarden/tensorflow,cxxgtxy/tensorflow,Intel-tensorflow/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_saved_model,karllessard/tensorflow,Intel-tensorflow/tensorflow,gunan/tensorflow,freedomtan/tensorflow,arborh/tensorflow,aam-at/tensorflow,jhseu/tensorflow,petewarden/tensorflow,sarvex/tensorflow,tensorflow/tensorflow,renyi533/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_saved_model,davidzchen/tensorflow,aldian/tensorflow,renyi533/tensorflow,jhseu/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,paolodedios/tensorflow,ppwwyyxx/tensorflow,karllessard/tensorflow,paolodedios/tensorflow,Intel-Corporation/tensorflow,xzturn/tensorflow,paolodedios/tensorflow,xzturn/tensorflow,paolodedios/tensorflow,xzturn/tensorflow,xzturn/tensorflow,gautam1858/tensorflow,ppwwyyxx/tensorflow,yongtang/tensorflow,gunan/tensorflow,sarvex/tensorflow,gautam1858/tensorflow,yongtang/tensorflow,Intel-tensorflow/tensorflow,sarvex/tensorflow,jhseu/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,freedomtan/tensorflow,renyi533/tensorflow,jhseu/tensorflow,yongtang/tensorflow,gunan/tensorflow,sarvex/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gautam1858/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,yongtang/tensorflow,aam-at/tensorflow,freedomtan/tensorflow,ppwwyyxx/tensorflow,annarev/tensorflow,ppwwyyxx/tensorflow,aldian/tensorflow,renyi533/tensorflow,adit-chandra/tensorflow,davidzchen/tensorflow,davidzchen/tensorflow,aam-at/tensorflow,frreiss/tensorflow-fred,aam-at/tensorflow,gautam1858/tensorflow,annarev/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,aam-at/tensorflow,paolodedios/tensorflow,freedomtan/tensorflow,sarvex/tensorflow,petewarden/tensorflow,davidzchen/tensorflow,yongtang/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_saved_model,freedomtan/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,karllessard/tensorflow,annarev/tensorflow,aam-at/tensorflow,annarev/tensorflow,cxxgtxy/tensorflow,davidzchen/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,freedomtan/tensorflow,annarev/tensorflow,karllessard/tensorflow,davidzchen/tensorflow,annarev/tensorflow,sarvex/tensorflow,jhseu/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,davidzchen/tensorflow,aam-at/tensorflow,jhseu/tensorflow,cxxgtxy/tensorflow,yongtang/tensorflow,tensorflow/tensorflow-pywrap_saved_model,petewarden/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,aldian/tensorflow,paolodedios/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,gunan/tensorflow,Intel-Corporation/tensorflow,davidzchen/tensorflow,ppwwyyxx/tensorflow,aldian/tensorflow,cxxgtxy/tensorflow,adit-chandra/tensorflow,aam-at/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow,arborh/tensorflow,Intel-Corporation/tensorflow,davidzchen/tensorflow,gunan/tensorflow,arborh/tensorflow,yongtang/tensorflow,Intel-Corporation/tensorflow,xzturn/tensorflow,frreiss/tensorflow-fred,Intel-tensorflow/tensorflow,gautam1858/tensorflow,xzturn/tensorflow,arborh/tensorflow,Intel-Corporation/tensorflow,yongtang/tensorflow,ppwwyyxx/tensorflow,adit-chandra/tensorflow,petewarden/tensorflow,tensorflow/tensorflow-pywrap_saved_model,davidzchen/tensorflow,xzturn/tensorflow,frreiss/tensorflow-fred,Intel-tensorflow/tensorflow,gautam1858/tensorflow,tensorflow/tensorflow,tensorflow/tensorflow-pywrap_saved_model,xzturn/tensorflow,petewarden/tensorflow,gunan/tensorflow,tensorflow/tensorflow-pywrap_saved_model,frreiss/tensorflow-fred,renyi533/tensorflow,adit-chandra/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,jhseu/tensorflow,annarev/tensorflow,adit-chandra/tensorflow,renyi533/tensorflow,ppwwyyxx/tensorflow,tensorflow/tensorflow-experimental_link_static_libraries_once,tensorflow/tensorflow-pywrap_saved_model,aldian/tensorflow,frreiss/tensorflow-fred,frreiss/tensorflow-fred,gunan/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,tensorflow/tensorflow-pywrap_tf_optimizer,annarev/tensorflow,karllessard/tensorflow,karllessard/tensorflow,arborh/tensorflow,gunan/tensorflow,renyi533/tensorflow,petewarden/tensorflow,yongtang/tensorflow,ppwwyyxx/tensorflow,renyi533/tensorflow,petewarden/tensorflow,arborh/tensorflow,Intel-tensorflow/tensorflow,gunan/tensorflow,tensorflow/tensorflow-pywrap_saved_model,tensorflow/tensorflow,Intel-tensorflow/tensorflow,tensorflow/tensorflow-pywrap_tf_optimizer,arborh/tensorflow,davidzchen/tensorflow,frreiss/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