Previously, we were comparing the signed int `target_length` (returned by WideCharToMultiByte) to a size_t string length, resulting in a signed/unsigned comparison warning as follows:
```
windows\runner\utils.cpp(54,43): warning C4018: '>': signed/unsigned mismatch
```
WideCharToMultiByte returns:
* 0 on error
* the number of bytes written to the buffer pointed to by its fifth parameter, lpMultiByteStr, on success.
As a result it's safe to store the return value in an unsigned int, which eliminates the warning.
No changes to tests since this is dependent on end-user project settings/modifications and does not trigger a warning with default project settings.
Fixes: https://github.com/flutter/flutter/issues/134227
## Background
The Windows runner has a race at startup:
1. **Platform thread**: creates a hidden window
2. **Platform thread**: launches the Flutter engine
3. **UI/Raster threads**: renders the first frame
4. **Platform thread**: Registers a callback to show the window once the next frame has been rendered.
Steps 3 and 4 happen in parallel and it is possible for step 3 to complete before step 4 starts. In this scenario, the next frame callback is never called and the window is never shown.
As a result the `windows_startup_test`'s test, which [verifies that the "show window" callback is called](1f09a8662d/dev/integration_tests/windows_startup_test/windows/runner/flutter_window.cpp (L60-L64)), can flake if the first frame is rendered before the show window callback has been registered.
## Solution
This change makes the runner schedule a frame after it registers the next frame callback. If step 3 hasn't completed yet, this no-ops as a frame is already scheduled. If step 3 has already completed, a new frame will be rendered, which will call the next frame callback and show the window.
Part of https://github.com/flutter/flutter/issues/119415
See this thread for alternatives that were considered: https://github.com/flutter/engine/pull/42061#issuecomment-1550080722
