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}
diff --git a/webrtc/base/buffer.h b/webrtc/base/buffer.h
index 076fa08..bf2e9f3 100644
--- a/webrtc/base/buffer.h
+++ b/webrtc/base/buffer.h
@@ -172,7 +172,7 @@
   // 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