Add GN flag for an easier JNI generator upgrade.

Bug: b/293234089
Change-Id: I06dca333a806992cbff1b175f1b0875728a9f1fb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/315120
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Christoffer Jansson <jansson@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40518}

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 67c5603..661e36f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -138,6 +138,10 @@
   cflags = []
   ldflags = []
 
+  if (rtc_jni_generator_legacy_symbols) {
+    defines += [ "RTC_JNI_GENERATOR_LEGACY_SYMBOLS" ]
+  }
+
   if (rtc_objc_prefix != "") {
     defines += [ "RTC_OBJC_TYPE_PREFIX=${rtc_objc_prefix}" ]
   }