Use Vector::Contains() in SpaceSplitString::Data::Contains().

The original code of SpaceSplitString::Data::Contains() was equivalent to
Vector::Contains(). This CL simplifies the code.

This CL has no behavior changes.

Change-Id: I0ed0f68e87e07bb003a7b67159982600431618f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1852524Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704920}

third_party/blink/renderer/core/dom/space_split_string.h

@@ -74,12 +74,8 @@ class CORE_EXPORT SpaceSplitString {
 
     ~Data();
 
-    bool Contains(const AtomicString& string) {
-      for (const auto& item : vector_) {
-        if (item == string)
-          return true;
-      }
-      return false;
+    bool Contains(const AtomicString& string) const {
+      return vector_.Contains(string);
     }
 
     bool ContainsAll(Data&);