Use AtomicString for scope constants in measure_memory_controller

Bug: 1157095
Change-Id: I947cfdcd5796e4d19aaf0d3e03b6c9ca4aa7b759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2610782Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845257}

third_party/blink/renderer/core/timing/measure_memory/measure_memory_controller.cc

@@ -176,7 +176,7 @@ struct RandomBitGenerator {
 };
 
 // These functions convert WebMemory* mojo structs to IDL and JS values.
-WTF::String ConvertScope(WebMemoryAttribution::Scope scope) {
+WTF::AtomicString ConvertScope(WebMemoryAttribution::Scope scope) {
   using Scope = WebMemoryAttribution::Scope;
   switch (scope) {
     case Scope::kDedicatedWorker: