Fix //third_party/r8/playground/build.sh

Our version of R8 no longer works with java -jar

Bug: None
Change-Id: Ida233fed73b2be6f65a54a292efbda6a1c2ec4cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426772
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810200}

third_party/r8/playground/build.sh

@@ -14,5 +14,6 @@ DEXDUMP=${dexdumps[0]}
 
 rm -f *.class
 javac *.java && \
-java -jar ../lib/r8.jar *.class --output . --lib ../../jdk/current --no-minification --pg-conf playground.pgcfg && \
+java -cp ../lib/r8.jar com.android.tools.r8.R8 *.class --output . --lib ../../jdk/current --no-minification --pg-conf playground.pgcfg && \
 $DEXDUMP -d classes.dex > dexdump.txt
+echo 'dexdump.txt updated.'