Update highestSortingString atomically.

R=bak@google.com

Bug: 63745863
Change-Id: Id9d83f2abe1b7e67c1a5f6744fc72fedea0ea8c6

src/main/java/com/android/tools/r8/ir/conversion/IRConverter.java

diff --git a/src/main/java/com/android/tools/r8/ir/conversion/IRConverter.java b/src/main/java/com/android/tools/r8/ir/conversion/IRConverter.java
index 98a2619..ba2b425 100644
--- a/src/main/java/com/android/tools/r8/ir/conversion/IRConverter.java
+++ b/src/main/java/com/android/tools/r8/ir/conversion/IRConverter.java
@@ -550,7 +550,7 @@
     feedback.markProcessed(method, state);
   }
 
-  private void updateHighestSortingStrings(DexEncodedMethod method) {
+  private synchronized void updateHighestSortingStrings(DexEncodedMethod method) {
     DexString highestSortingReferencedString = method.getCode().asDexCode().highestSortingString;
     if (highestSortingReferencedString != null) {
       if (highestSortingString == null