Base start bitrate on last observed bitrate.

Instead of setting bitrates based on codec target settings (which may
have previously been capped by a codec max bitrate), fetch the last
bandwidth allocated for this channel. This fixes broken low start bitrates
due to QCIF being set as default codec in WebRtcVideoEngine2 which caps
the max bitrate to 200kbps.

BUG=1788
R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43789004

Cr-Commit-Position: refs/heads/master@{#8780}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8780 4adac7df-926f-26a2-2b94-8c16560cd09d

talk/media/webrtc/fakewebrtcvideoengine.h

@@ -831,6 +831,9 @@ class FakeWebRtcVideoEngine
       unsigned int&, unsigned int&));
   WEBRTC_STUB_CONST(GetReceiveSideDelay, (const int video_channel,
                                           int* delay_ms));
+  virtual uint32_t GetLastObservedBitrateBps(int channel) const override {
+    return 0;
+  }
   WEBRTC_FUNC_CONST(GetCodecTargetBitrate, (const int channel,
       unsigned int* codec_target_bitrate)) {
     WEBRTC_CHECK_CHANNEL(channel);