Delete FrameObject::size member.

Replaced with a size() method, returning the corresponding attribute
(_length) of the underlying EncodedImage.

BUG=None

Review-Url: https://codereview.webrtc.org/2444193010
Cr-Commit-Position: refs/heads/master@{#14809}
diff --git a/webrtc/modules/video_coding/frame_buffer2.cc b/webrtc/modules/video_coding/frame_buffer2.cc
index 8bb19ba..daac7d5 100644
--- a/webrtc/modules/video_coding/frame_buffer2.cc
+++ b/webrtc/modules/video_coding/frame_buffer2.cc
@@ -119,7 +119,7 @@
     int64_t frame_delay;
     if (inter_frame_delay_.CalculateDelay(timestamp, &frame_delay,
                                           received_time)) {
-      jitter_estimator_->UpdateEstimate(frame_delay, frame->size);
+      jitter_estimator_->UpdateEstimate(frame_delay, frame->size());
     }
     float rtt_mult = protection_mode_ == kProtectionNackFEC ? 0.0 : 1.0;
     timing_->SetJitterDelay(jitter_estimator_->GetJitterEstimate(rtt_mult));