Use helpers in ShaderVariable

Facilitates renaming a variable in that class inside ANGLE.

Bug: angleproject:3580
Change-Id: I5b00e1b72ada0aa77f9276f7af28b33518e77d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637956
Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845258}

gpu/command_buffer/service/memory_program_cache.cc

@@ -43,7 +43,7 @@ void FillShaderVariableProto(
     ShaderVariableProto* field = proto->add_fields();
     FillShaderVariableProto(field, variable.fields[ii]);
   }
-  proto->set_struct_name(variable.structName);
+  proto->set_struct_name(variable.getStructName());
 }
 
 void FillShaderAttributeProto(
@@ -134,7 +134,7 @@ void RetrieveShaderVariableInfo(
   variable->fields.resize(proto.fields_size());
   for (int ii = 0; ii < proto.fields_size(); ++ii)
     RetrieveShaderVariableInfo(proto.fields(ii), &(variable->fields[ii]));
-  variable->structName = proto.struct_name();
+  variable->setStructName(proto.struct_name());
 }
 
 void RetrieveShaderAttributeInfo(