Fix crash during screen capture using WGC on Windows

WgcCaptureSession::WgcCaptureSession previously assumed the input
source ID would never be 0, based on the expectation that it only
captures windows where the source ID is an HWND, which is non-zero.

However, Chromium also uses WgcCaptureSession for screen capture,
where the source ID is a 0-based index.

This change removes the incorrect source index check. Instead,
it evaluates whether the source index refers to a window or a screen
and uses the appropriate monitor retrieval API in the ProcessFrame() method.

Bug: chromium:404887529
Change-Id: I43e3b77b8d2c46fd201cc178113a9860aadba921
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/382101
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Palak Agarwal <agpalak@google.com>
Commit-Queue: Sunggook Chue <sunggch@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#44221}