Added an empty() method to rtc::Buffer

I'll be using it in a separate CL in a bit.

Review-Url: https://codereview.webrtc.org/2290343002
Cr-Commit-Position: refs/heads/master@{#14001}
diff --git a/webrtc/base/buffer.h b/webrtc/base/buffer.h
index 545ddb2..e9f7cfa 100644
--- a/webrtc/base/buffer.h
+++ b/webrtc/base/buffer.h
@@ -124,6 +124,11 @@
     return reinterpret_cast<U*>(data_.get());
   }
 
+  bool empty() const {
+    RTC_DCHECK(IsConsistent());
+    return size_ == 0;
+  }
+
   size_t size() const {
     RTC_DCHECK(IsConsistent());
     return size_;