Move base64 response encoding into BinaryResult

Make it easy for any RestApiView to return binary data within a
base64 wrapper by marking a BinaryResult with base64() before it
is given to RestApiServlet. The servlet will process the base64
wrapping before gzip encoding, which may save on transfer cost.

Change-Id: I5ed7b8cb2b034b60654cc2574e627159b11a4f27

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/restapi/BinaryResult.java

@@ -61,6 +61,7 @@ public abstract class BinaryResult implements Closeable {
   private String characterEncoding;
   private long contentLength = -1;
   private boolean gzip = true;
+  private boolean base64 = false;
 
   /** @return the MIME type of the result, for HTTP clients. */
   public String getContentType() {
@@ -110,6 +111,17 @@ public abstract class BinaryResult implements Closeable {
     return this;
   }
 
+  /** @return true if the result must be base64 encoded. */
+  public boolean isBase64() {
+    return base64;
+  }
+
+  /** Wrap the binary data in base64 encoding. */
+  public BinaryResult base64() {
+    base64 = true;
+    return this;
+  }
+
   /**
    * Write or copy the result onto the specified output stream.
    *