Expand WTF::StringView's API to be more like StringPiece.

third_party/WebKit/Source/core/testing/UnionTypesTest.cpp

Index: third_party/WebKit/Source/core/testing/UnionTypesTest.cpp
diff --git a/third_party/WebKit/Source/core/testing/UnionTypesTest.cpp b/third_party/WebKit/Source/core/testing/UnionTypesTest.cpp
index 3e134f78931103f0b586b1a33ea28b118603cc85..ebb3aabc6dc3325fc079a31673603518585f1427 100644
--- a/third_party/WebKit/Source/core/testing/UnionTypesTest.cpp
+++ b/third_party/WebKit/Source/core/testing/UnionTypesTest.cpp
@@ -75,12 +75,15 @@ String UnionTypesTest::doubleOrStringArrayArg(HeapVector<DoubleOrString>& array)
     StringBuilder builder;
     for (DoubleOrString& doubleOrString : array) {
         ASSERT(!doubleOrString.isNull());
-        if (doubleOrString.isDouble())
-            builder.append("double: " + String::numberToStringECMAScript(doubleOrString.getAsDouble()));
-        else if (doubleOrString.isString())
-            builder.append("string: " + doubleOrString.getAsString());
-        else
+        if (doubleOrString.isDouble()) {
+            builder.append("double: ");
+            builder.append(String::numberToStringECMAScript(doubleOrString.getAsDouble()));
+        } else if (doubleOrString.isString()) {
+            builder.append("string: ");
+            builder.append(doubleOrString.getAsString());
+        } else {
             ASSERT_NOT_REACHED();
+        }
         builder.append(", ");
     }
     return builder.substring(0, builder.length() - 2);