Introduce a Blink API for checking whether a certain locale could be valid.

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index b8598e36665f9128c1a0cc67a3f998547a213b9f..afa29267bbad7fb2656bfdad6bc0610000b96357 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -432,6 +432,13 @@ public:
     // Returns an interface to the main thread. Can be null if blink was initialized on a thread without a message loop.
     BLINK_PLATFORM_EXPORT WebThread* mainThread() const;
 
+
+    // Localisation --------------------------------------------------------
+
+    // Returns if |locale| is plausibly a syntactically-valid locale string based on BCP 47.
+    virtual bool isValidLocaleSyntax(const WebString& locale) { return true; }
+
+
     // Vibration -----------------------------------------------------------
 
     // Starts a vibration for the given duration in milliseconds. If there is currently an active