Relanding "Make StringConsumer.finished an empty default method."

This cherry-picks 8243cf55dcd0b847f918f460c586055819159d5b.

The change was reverted as part of 523ef9455bab0bd029227982f0ade905d710feeb.

Bug: 140141590
Change-Id: I9d533983f88629be686642014b0f1831fbd0ae54

src/main/java/com/android/tools/r8/StringConsumer.java

diff --git a/src/main/java/com/android/tools/r8/StringConsumer.java b/src/main/java/com/android/tools/r8/StringConsumer.java
index 5d02846..6e440eb 100644
--- a/src/main/java/com/android/tools/r8/StringConsumer.java
+++ b/src/main/java/com/android/tools/r8/StringConsumer.java
@@ -41,7 +41,7 @@
    *
    * @param handler Diagnostics handler for reporting.
    */
-  void finished(DiagnosticsHandler handler);
+  default void finished(DiagnosticsHandler handler) {}
 
   static EmptyConsumer emptyConsumer() {
     return EmptyConsumer.EMPTY_CONSUMER;