desktopCapture: fix regression of handling minimized shared window.

We receive a 1x1 frame when the shared window is minimized and will replace it
with a blackframe to avaoid video freezing at last frame. But due to frame size
rounding added later, the 1x1 frame will be dropped. Let's fix it again by
replacing it with a 2x2 black frame.

Bug: 568835
Change-Id: Ifd9a37b62e4f179318c8b30bee4687494d976ab1
Reviewed-on: https://chromium-review.googlesource.com/596920Reviewed-by: Zijie He <zijiehe@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491533}

content/browser/media/capture/desktop_capture_device.cc

@@ -267,11 +267,14 @@ void DesktopCaptureDevice::Core::OnCaptureResult(
   }
   // Align to 2x2 pixel boundaries, as required by OnIncomingCapturedData() so
   // it can convert the frame to I420 format.
-  const webrtc::DesktopSize output_size(
+  webrtc::DesktopSize output_size(
       resolution_chooser_->capture_size().width() & ~1,
       resolution_chooser_->capture_size().height() & ~1);
-  if (output_size.is_empty())
-    return;
+  if (output_size.is_empty()) {
+    // Even RESOLUTION_POLICY_ANY_WITHIN_LIMIT is used, a non-empty size should
+    // be guaranteed.
+    output_size.set(2, 2);
+  }
 
   size_t output_bytes = output_size.width() * output_size.height() *
       webrtc::DesktopFrame::kBytesPerPixel;