Replace manual casting to rvalue reference with calls to std::move

Review URL: https://codereview.webrtc.org/1570473002

Cr-Commit-Position: refs/heads/master@{#11163}

webrtc/base/buffer.h

@@ -172,7 +172,7 @@ class Buffer {
   // b.Pass() does the same thing as std::move(b).
   Buffer&& Pass() {
     assert(IsConsistent());
-    return static_cast<Buffer&&>(*this);
+    return std::move(*this);
   }
 
   // Resets the buffer to zero size and capacity. Works even if the buffer has