Refactor scaling.

Introduce a new method I420Buffer::CropAndScale, and a static
convenience helper I420Buffer::CenterCropAndScale. Use them for almost
all scaling needs.

Delete the Scaler class and the cricket::VideoFrame::Stretch* methods.

BUG=webrtc:5682
R=pbos@webrtc.org, perkj@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2020593002 .

Cr-Commit-Position: refs/heads/master@{#13110}
diff --git a/webrtc/api/java/jni/androidmediaencoder_jni.cc b/webrtc/api/java/jni/androidmediaencoder_jni.cc
index a240b25..b76cfe7 100644
--- a/webrtc/api/java/jni/androidmediaencoder_jni.cc
+++ b/webrtc/api/java/jni/androidmediaencoder_jni.cc
@@ -670,7 +670,7 @@
   VideoFrame input_frame = frame;
   if (scale_) {
     // Check framerate before spatial resolution change.
-    quality_scaler_.OnEncodeFrame(frame);
+    quality_scaler_.OnEncodeFrame(frame.width(), frame.height());
     const webrtc::QualityScaler::Resolution scaled_resolution =
         quality_scaler_.GetScaledResolution();
     if (scaled_resolution.width != frame.width() ||
@@ -684,7 +684,8 @@
                     webrtc::kVideoRotation_0));
         input_frame.set_video_frame_buffer(scaled_buffer);
       } else {
-        input_frame = quality_scaler_.GetScaledFrame(frame);
+        input_frame.set_video_frame_buffer(
+            quality_scaler_.GetScaledBuffer(frame.video_frame_buffer()));
       }
     }
   }