Make jni_helpers build on arm32.

BUG=None

Review URL: https://codereview.webrtc.org/1311753002

Cr-Commit-Position: refs/heads/master@{#9806}

talk/app/webrtc/java/jni/jni_helpers.cc

@@ -103,7 +103,8 @@ jint InitGlobalJniVariables(JavaVM *jvm) {
 // Return thread ID as a string.
 static std::string GetThreadId() {
   char buf[21];  // Big enough to hold a kuint64max plus terminating NULL.
-  CHECK_LT(snprintf(buf, sizeof(buf), "%ld", syscall(__NR_gettid)),
+  CHECK_LT(snprintf(buf, sizeof(buf), "%ld",
+           static_cast<long>(syscall(__NR_gettid))),
            sizeof(buf))
       << "Thread id is bigger than uint64??";
   return std::string(buf);