Fix problem when drawing to the Android Media encoder surface.
Problem seen on N6.
BUG=webrtc:5147

TBR=magjed@webrtc.org

Review URL: https://codereview.webrtc.org/1491623003

Cr-Commit-Position: refs/heads/master@{#10866}
diff --git a/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java b/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
index f10df98..58709ea 100644
--- a/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
+++ b/talk/app/webrtc/java/src/org/webrtc/MediaCodecVideoEncoder.java
@@ -380,6 +380,9 @@
         mediaCodec.setParameters(b);
       }
       eglBase.makeCurrent();
+      // TODO(perkj): glClear() shouldn't be necessary since every pixel is covered anyway,
+      // but it's a workaround for bug webrtc:5147.
+      GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);
       drawer.drawOes(oesTextureId, transformationMatrix);
       // TODO(perkj): Do we have to call EGLExt.eglPresentationTimeANDROID ?
       // If not, remove |presentationTimestampUs|.