8261880: Change nested classes in java.base to static nested classes where possible

Reviewed-by: redestad

src/java.base/share/classes/java/lang/StackStreamFactory.java

@@ -620,7 +620,7 @@ final class StackStreamFactory {
             super(walker, FILL_CLASS_REFS_ONLY|GET_CALLER_CLASS);
         }
 
-        final class ClassBuffer extends FrameBuffer<Class<?>> {
+        static final class ClassBuffer extends FrameBuffer<Class<?>> {
             Class<?>[] classes;      // caller class for fast path
             ClassBuffer(int batchSize) {
                 super(batchSize);

src/java.base/share/classes/java/net/Inet6Address.java

@@ -176,7 +176,7 @@ public final
 class Inet6Address extends InetAddress {
     static final int INADDRSZ = 16;
 
-    private class Inet6AddressHolder {
+    private static class Inet6AddressHolder {
 
         private Inet6AddressHolder() {
             ipaddress = new byte[INADDRSZ];

src/java.base/share/classes/java/security/ProtectionDomain.java

@@ -566,6 +566,6 @@ public class ProtectionDomain {
     /**
      * Used for storing ProtectionDomains as keys in a Map.
      */
-    final class Key {}
+    static final class Key {}
 
 }