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/app/webrtc/datachannel_unittest.cc b/talk/app/webrtc/datachannel_unittest.cc
index 6a73fbc..ab5dbe9 100644
--- a/talk/app/webrtc/datachannel_unittest.cc
+++ b/talk/app/webrtc/datachannel_unittest.cc
@@ -145,7 +145,7 @@
   for (int i = 0; i < number_of_packets; ++i) {
     EXPECT_TRUE(webrtc_data_channel_->Send(buffer));
   }
-  EXPECT_EQ(buffer.data.length() * number_of_packets,
+  EXPECT_EQ(buffer.data.size() * number_of_packets,
             webrtc_data_channel_->buffered_amount());
 }
 
@@ -359,10 +359,8 @@
 TEST_F(SctpDataChannelTest, ClosedWhenSendBufferFull) {
   SetChannelReady();
 
-  const size_t buffer_size = 1024;
-  rtc::Buffer buffer;
-  buffer.SetLength(buffer_size);
-  memset(buffer.data(), 0, buffer_size);
+  rtc::Buffer buffer(1024);
+  memset(buffer.data(), 0, buffer.size());
 
   webrtc::DataBuffer packet(buffer, true);
   provider_.set_send_blocked(true);
@@ -413,10 +411,8 @@
 // Tests that the DataChannel is closed if the received buffer is full.
 TEST_F(SctpDataChannelTest, ClosedWhenReceivedBufferFull) {
   SetChannelReady();
-  const size_t buffer_size = 1024;
-  rtc::Buffer buffer;
-  buffer.SetLength(buffer_size);
-  memset(buffer.data(), 0, buffer_size);
+  rtc::Buffer buffer(1024);
+  memset(buffer.data(), 0, buffer.size());
 
   cricket::ReceiveDataParams params;
   params.ssrc = 0;