jni_helpers: Optimize IsNull()

The current implementation is unnecessary expensive - we create a local reference frame for creating new Java objects and then create a new local reference. It's cheaper to just do jni->IsSameObject(obj, nullptr).

R=perkj@webrtc.org

Review URL: https://codereview.webrtc.org/1741723002 .

Cr-Commit-Position: refs/heads/master@{#11825}

webrtc/api/java/jni/jni_helpers.cc

@@ -202,11 +202,8 @@ bool GetBooleanField(JNIEnv* jni, jobject object, jfieldID id) {
   return b;
 }
 
-// Java references to "null" can only be distinguished as such in C++ by
-// creating a local reference, so this helper wraps that logic.
 bool IsNull(JNIEnv* jni, jobject obj) {
-  ScopedLocalRefFrame local_ref_frame(jni);
-  return jni->NewLocalRef(obj) == NULL;
+  return jni->IsSameObject(obj, nullptr);
 }
 
 // Given a UTF-8 encoded |native| string return a new (UTF-16) jstring.

webrtc/api/java/jni/jni_helpers.h

@@ -72,8 +72,7 @@ jint GetIntField(JNIEnv* jni, jobject object, jfieldID id);
 
 bool GetBooleanField(JNIEnv* jni, jobject object, jfieldID id);
 
-// Java references to "null" can only be distinguished as such in C++ by
-// creating a local reference, so this helper wraps that logic.
+// Returns true if |obj| == null in Java.
 bool IsNull(JNIEnv* jni, jobject obj);
 
 // Given a UTF-8 encoded |native| string return a new (UTF-16) jstring.