DCHECK that jstring to C++ string conversions don't pass in null.

Also, update existing callers to avoid the call if they would otherwise trip the DCHECK now.

BUG=597564

Review URL: https://codereview.chromium.org/1828193002

Cr-Commit-Position: refs/heads/master@{#384010}
diff --git a/chrome/browser/ui/android/context_menu_helper.h b/chrome/browser/ui/android/context_menu_helper.h
index 7b77463..0455114 100644
--- a/chrome/browser/ui/android/context_menu_helper.h
+++ b/chrome/browser/ui/android/context_menu_helper.h
@@ -35,7 +35,7 @@
   void OnStartDownload(JNIEnv* env,
                        const base::android::JavaParamRef<jobject>& obj,
                        jboolean jis_link,
-                       const base::android::JavaParamRef<jstring>& jheaders);
+                       jboolean jis_data_reduction_proxy_enabled);
   void SearchForImage(JNIEnv* env,
                       const base::android::JavaParamRef<jobject>& obj);
   void ShareImage(JNIEnv* env, const base::android::JavaParamRef<jobject>& obj);