Allow null exception reporter.

libchrome in ARC++ runs without a java exception reporter. Logcat will
dump the information after an exception happens.

Bug: b:164205917
Test: build.
Change-Id: Id07dda246b8e63b52033cb961245281caa898b48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407252
Commit-Queue: Garfield Tan <xutan@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806816}

base/android/java_exception_reporter.cc

@@ -61,8 +61,11 @@ void SetJavaExceptionCallback(void (*callback)(const char*)) {
 }
 
 void SetJavaException(const char* exception) {
-  DCHECK(g_java_exception_callback);
-  g_java_exception_callback(exception);
+  // No need to print exception because they are already logged via
+  // env->ExceptionDescribe() within jni_android.cc.
+  if (g_java_exception_callback) {
+    g_java_exception_callback(exception);
+  }
 }
 
 void JNI_JavaExceptionReporter_ReportJavaException(