Initialize next_session_id_ in ProxyMediaKeys.

content/renderer/media/crypto/proxy_media_keys.cc

Index: content/renderer/media/crypto/proxy_media_keys.cc
diff --git a/content/renderer/media/crypto/proxy_media_keys.cc b/content/renderer/media/crypto/proxy_media_keys.cc
index 551570ce6474603423238bc7d4024b68092514c3..68a68b3bc370f6ca5d00f759da469ad6172fcb20 100644
--- a/content/renderer/media/crypto/proxy_media_keys.cc
+++ b/content/renderer/media/crypto/proxy_media_keys.cc
@@ -210,7 +210,8 @@ ProxyMediaKeys::ProxyMediaKeys(
       session_message_cb_(session_message_cb),
       session_ready_cb_(session_ready_cb),
       session_closed_cb_(session_closed_cb),
-      session_error_cb_(session_error_cb) {
+      session_error_cb_(session_error_cb),
+      next_session_id_(1) {
   cdm_id_ = manager->RegisterMediaKeys(this);
 }