EglRenderer: Clear texture before drawing a new frame.
EglRenderer: Clear texture before drawing a new frame.

This is necessary in case the drawer doesn't cover all the pixels.

BUG=None

Review-Url: https://codereview.webrtc.org/2704663002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16671}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 103988d040b4d7b2e6deb9886e5f23052fe36e38

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk