Expose the option of try_best_compression_for_png

pagespeed/kernel/image/image_optimizer.cc

@@ -230,7 +230,7 @@ bool ImageOptimizer::ConfigureWriter() {
     case IMAGE_PNG:
       png_config.reset(
           new PngCompressParams(
-              true /* try best compression */,
+              options_.try_best_compression_for_png(),
               false /* never use progressive format */));
       writer_config_.reset(png_config.release());
       result = true;

pagespeed/kernel/image/image_optimizer.proto

@@ -35,6 +35,10 @@ message ImageOptions {
   optional int32 max_webp_quality = 9 [ default = 85 ];
   optional int32 max_webp_animated_quality = 10 [ default = 85 ];
   optional int32 max_timeout_ms = 11 [ default = -1 ];
+  // When |try_best_compression_for_png| is set to true, the optimizer uses
+  // multiple methods, including the expensive ones, for encoding PNG images.
+  // The smallest output from all methods will be returned.
+  optional bool try_best_compression_for_png = 12 [ default = true ];
 }
 
 message ImageDimensions {