Have CSSFontFeatureValue store the tag as an AtomicString instead of a String

Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp

Index: Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp
diff --git a/Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp b/Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp
index f6c6da71ced11496d78c6dbb893934fc946f8bfe..c0d30992c9dc9a639656aef205a7a9275c0f3ee4 100644
--- a/Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp
+++ b/Source/core/platform/graphics/harfbuzz/HarfBuzzShaper.cpp
@@ -527,7 +527,7 @@ void HarfBuzzShaper::setFontFeatures()
     unsigned numFeatures = settings->size();
     for (unsigned i = 0; i < numFeatures; ++i) {
         hb_feature_t feature;
-        String tag = settings->at(i).tag();
+        const AtomicString& tag = settings->at(i).tag();
         feature.tag = HB_TAG(tag[0], tag[1], tag[2], tag[3]);
         feature.value = settings->at(i).value();
         feature.start = 0;