Use unordered_set<std::string> for deprecation warnings

This fixes the roll in Google3, otherwise it complains that
unordered_set<const char*> hashes the pointer, which is most likely not
what you want to do.

Bug: dawn:394
Change-Id: I1ef06c923bb45e55b39c23e5e7734f1577c47255
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/20860
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/Device.cpp

@@ -74,7 +74,7 @@ namespace dawn_native {
     };
 
     struct DeviceBase::DeprecationWarnings {
-        std::unordered_set<const char*> emitted;
+        std::unordered_set<std::string> emitted;
         size_t count = 0;
     };