Resuse the gson instance when parsing source maps

Gson object are kind of expensive to construct (due to the thread local instances).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=239489982

src/com/google/debugging/sourcemap/SourceMapObjectParser.java

@@ -29,12 +29,17 @@
  * Java implementation of the source map parser.
  */
 public class SourceMapObjectParser {
+  // Gson objects are safe to share across threads and are somewhat expensive to construct. Use a
+  // single instance.  This is Safe per gson docs:
+  // https://google.github.io/gson/apidocs/com/google/gson/Gson.html
+  private static final Gson gson = new Gson();
+
   public static SourceMapObject parse(String contents) throws SourceMapParseException {
 
     SourceMapObject.Builder builder = SourceMapObject.builder();
 
     try {
-      JsonObject sourceMapRoot = new Gson().fromJson(contents, JsonObject.class);
+      JsonObject sourceMapRoot = gson.fromJson(contents, JsonObject.class);
 
       builder.setVersion(sourceMapRoot.get("version").getAsInt());
       builder.setFile(getStringOrNull(sourceMapRoot, "file"));