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.

See https://webrtc-review.googlesource.com/c/src/+/97184/

Bug: webrtc:9703
Change-Id: I27d096058ead075765f4c49bf60cb8d725da1afd
Reviewed-on: https://webrtc-review.googlesource.com/c/120700
Commit-Queue: Brave Yao <braveyao@webrtc.org>
Reviewed-by: Brave Yao <braveyao@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26504}

modules/desktop_capture/cropped_desktop_frame_unittest.cc

@@ -64,4 +64,17 @@ TEST(CroppedDesktopFrameTest, SetTopLeft) {
   ASSERT_EQ(frame->top_left().y(), 203);
 }
 
+TEST(CroppedDesktopFrameTest, InitializedWithZeros) {
+  std::unique_ptr<DesktopFrame> frame = CreateTestFrame();
+  const DesktopVector frame_origin = frame->top_left();
+  const DesktopSize frame_size = frame->size();
+  std::unique_ptr<DesktopFrame> cropped = CreateCroppedDesktopFrame(
+      std::move(frame), DesktopRect::MakeOriginSize(frame_origin, frame_size));
+  for (int j = 0; j < cropped->size().height(); ++j) {
+    for (int i = 0; i < cropped->stride(); ++i) {
+      ASSERT_EQ(cropped->data()[i + j * cropped->stride()], 0);
+    }
+  }
+}
+
 }  // namespace webrtc

modules/desktop_capture/desktop_and_cursor_composer_unittest.cc

@@ -126,7 +126,6 @@ class FakeMouseMonitor : public MouseCursorMonitor {
       std::unique_ptr<DesktopFrame> image(
           new BasicDesktopFrame(DesktopSize(kCursorWidth, kCursorHeight)));
       uint32_t* data = reinterpret_cast<uint32_t*>(image->data());
-      memset(data, 0, image->stride() * kCursorHeight);
 
       // Set four pixels near the hotspot and leave all other blank.
       for (int y = 0; y < kTestCursorSize; ++y) {

modules/desktop_capture/desktop_frame.h

@@ -138,6 +138,7 @@ class RTC_EXPORT DesktopFrame {
 // A DesktopFrame that stores data in the heap.
 class RTC_EXPORT BasicDesktopFrame : public DesktopFrame {
  public:
+  // The entire data buffer used for the frame is initialized with zeros.
   explicit BasicDesktopFrame(DesktopSize size);
 
   ~BasicDesktopFrame() override;

modules/desktop_capture/win/dxgi_frame.cc

@@ -41,20 +41,22 @@ bool DxgiFrame::Prepare(DesktopSize size, DesktopCapturer::SourceId source_id) {
     std::unique_ptr<DesktopFrame> frame;
     if (factory_) {
       frame = SharedMemoryDesktopFrame::Create(size, factory_);
-    } else {
-      frame.reset(new BasicDesktopFrame(size));
-    }
+
       if (!frame) {
         RTC_LOG(LS_WARNING) << "DxgiFrame cannot create a new DesktopFrame.";
         return false;
       }
+
       // DirectX capturer won't paint each pixel in the frame due to its one
-    // capturer per monitor design. So once the new frame is created, we should
-    // clear it to avoid the legacy image to be remained on it. See
+      // capturer per monitor design. So once the new frame is created, we
+      // should clear it to avoid the legacy image to be remained on it. See
       // http://crbug.com/708766.
       RTC_DCHECK_EQ(frame->stride(),
                     frame->size().width() * DesktopFrame::kBytesPerPixel);
       memset(frame->data(), 0, frame->stride() * frame->size().height());
+    } else {
+      frame.reset(new BasicDesktopFrame(size));
+    }
 
     frame_ = SharedDesktopFrame::Wrap(std::move(frame));
   }

modules/desktop_capture/window_capturer_win.cc

@@ -249,7 +249,6 @@ void WindowCapturerWin::CaptureFrame() {
       !window_capture_helper_.IsWindowVisibleOnCurrentDesktop(window_)) {
     std::unique_ptr<DesktopFrame> frame(
         new BasicDesktopFrame(DesktopSize(1, 1)));
-    memset(frame->data(), 0, frame->stride() * frame->size().height());
 
     previous_size_ = frame->size();
     window_size_map_[window_] = previous_size_;