Don't use substring() for a simple prefix match

The language code match in SVGTests::isValid() uses String::substring()
to limit the match to the primary language subtag. In the case where the
defaultLanguage() only contains the/a primary language subtag, this will
not require a copy, but otherwise it would.

Using startsWith() and a length-check guarantees that no copies will be
made. (A valid primary language subtag is always 2 letter long, so
checking only for length == 2 should be enough, although previously the
code could theoretically match a 1 letter, or even a zero-length tag.)

Review-Url: https://codereview.chromium.org/2284273002
Cr-Commit-Position: refs/heads/master@{#415255}

third_party/WebKit/Source/core/svg/SVGTests.cpp

@@ -51,15 +51,12 @@ bool SVGTests::isValid() const
 
     if (m_systemLanguage->isSpecified()) {
         bool matchFound = false;
-
-        const Vector<String>& systemLanguage = m_systemLanguage->value()->values();
-        for (const auto& value : systemLanguage) {
-            if (value == defaultLanguage().getString().substring(0, 2)) {
+        for (const auto& value : m_systemLanguage->value()->values()) {
+            if (value.length() == 2 && defaultLanguage().startsWith(value)) {
                 matchFound = true;
                 break;
             }
         }
-
         if (!matchFound)
             return false;
     }