rtc::Buffer: Rename length to size, for conformance with the STL

And add a constructor for creating an uninitialized Buffer of a
specified size.

(I intend to follow up with more Buffer changes, but since it's rather
widely used, the rename is quite noisy and works better as a separate
CL.)

R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48579004

Cr-Commit-Position: refs/heads/master@{#8841}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8841 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/talk/media/base/rtpdataengine_unittest.cc b/talk/media/base/rtpdataengine_unittest.cc
index 884fab4..0cd1b2a 100644
--- a/talk/media/base/rtpdataengine_unittest.cc
+++ b/talk/media/base/rtpdataengine_unittest.cc
@@ -143,8 +143,8 @@
     // Assume RTP header of length 12
     rtc::scoped_ptr<const rtc::Buffer> packet(
         iface_->GetRtpPacket(index));
-    if (packet->length() > 12) {
-      return std::string(packet->data() + 12, packet->length() - 12);
+    if (packet->size() > 12) {
+      return std::string(packet->data() + 12, packet->size() - 12);
     } else {
       return "";
     }
@@ -154,7 +154,7 @@
     rtc::scoped_ptr<const rtc::Buffer> packet(
         iface_->GetRtpPacket(index));
     cricket::RtpHeader header;
-    GetRtpHeader(packet->data(), packet->length(), &header);
+    GetRtpHeader(packet->data(), packet->size(), &header);
     return header;
   }