desktopCapture: remove unnecessary memset to DesktopFrame

With a recent change, the webrtc::DesktopFrame is created and
initialized with 0. So it's not necessary to call memset() to the frame
again any more.

Bug: webrtc:9703
Change-Id: Id3b37940ad3a1e989725db578f743670c786e681
Reviewed-on: https://chromium-review.googlesource.com/c/1366785Reviewed-by: Julien Isorce <julien.isorce@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614733}

content/browser/media/capture/desktop_capture_device.cc

@@ -364,8 +364,6 @@ void DesktopCaptureDevice::Core::OnCaptureResult(
     // last frame.
     if (!black_frame_ || !black_frame_->size().equals(output_size)) {
       black_frame_.reset(new webrtc::BasicDesktopFrame(output_size));
-      memset(black_frame_->data(), 0,
-             black_frame_->stride() * black_frame_->size().height());
     }
     output_data = black_frame_->data();
   } else {
@@ -396,7 +394,6 @@ void DesktopCaptureDevice::Core::OnCaptureResult(
       // letterboxed areas.
       if (!output_frame_) {
         output_frame_.reset(new webrtc::BasicDesktopFrame(output_size));
-        memset(output_frame_->data(), 0, output_bytes);
       }
       DCHECK(output_frame_->size().equals(output_size));
 
@@ -418,7 +415,6 @@ void DesktopCaptureDevice::Core::OnCaptureResult(
       // crbug.com/437740).
       if (!output_frame_) {
         output_frame_.reset(new webrtc::BasicDesktopFrame(output_size));
-        memset(output_frame_->data(), 0, output_bytes);
       }
 
       output_frame_->CopyPixelsFrom(