Remove mutable from rtc::CriticalSection members.

webrtc/base/stream.h

Index: webrtc/base/stream.h
diff --git a/webrtc/base/stream.h b/webrtc/base/stream.h
index c57daae76c591ba83709da6c134317cd4b0237f4..98123b82118ee9f9c75b3b4aa0901ded4c610d84 100644
--- a/webrtc/base/stream.h
+++ b/webrtc/base/stream.h
@@ -555,7 +555,7 @@ class FifoBuffer : public StreamInterface {
   size_t data_length_;  // amount of readable data in the buffer
   size_t read_position_;  // offset to the readable data
   Thread* owner_;  // stream callbacks are dispatched on this thread
-  mutable CriticalSection crit_;  // object lock
+  CriticalSection crit_;  // object lock
   RTC_DISALLOW_COPY_AND_ASSIGN(FifoBuffer);
 };