Avoid manual updates to R8 playground script.

Avoid needing to update the R8 playground script to point at the correct
version of build-tools by just finding the first one available, since
this is just a quick experimentation tool and not part of the actual
build.

Change-Id: If277326101039b1e5ec2ef0541008045957ed398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316317
Auto-Submit: Richard Coles <torne@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791264}

third_party/r8/playground/build.sh

@@ -6,7 +6,13 @@
 # Usage:
 # Make edits to *.java and *.pgcfg
 # Then run: ./build.sh | less
+
+
+# Finds any version of dexdump available
+dexdumps=( ../../android_sdk/public/build-tools/*/dexdump )
+DEXDUMP=${dexdumps[0]}
+
 rm -f *.class
 javac *.java && \
 java -jar ../lib/r8.jar *.class --output . --lib ../../jdk/current --no-minification --pg-conf playground.pgcfg && \
-../../android_sdk/public/build-tools/29.0.2/dexdump -d classes.dex > dexdump.txt
+$DEXDUMP -d classes.dex > dexdump.txt