Change pdf_fm2js_fuzzer to be a "public" fuzzer.

This fuzzer does not use any APIs from public/, but it calls
FPDF_GetFuzzerPerProcessState(), and that function is effectively public
even though it is only exposed to fuzzers for testing purposes.

Bug: pdfium:1563
Change-Id: I9405b8a60b884c9b3ff97445a1fadbc716195914
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/73710
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

testing/fuzzers/BUILD.gn

diff --git a/testing/fuzzers/BUILD.gn b/testing/fuzzers/BUILD.gn
index bf9f05b..854b51d 100644
--- a/testing/fuzzers/BUILD.gn
+++ b/testing/fuzzers/BUILD.gn
@@ -344,6 +344,7 @@
         "../../core/fxcrt",
         "../../fxjs",
       ]
+      public_fuzzer = true
     }
 
     pdfium_fuzzer("pdf_formcalc_context_fuzzer") {