Make FrameGeneratorInterface::GetResolution pure virtual.

Bug: b/269577953
Change-Id: Ia8d370b9741fe3ed19ce276265ff7de7dcd061d8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/293961
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39335}

api/test/frame_generator_interface.h

diff --git a/api/test/frame_generator_interface.h b/api/test/frame_generator_interface.h
index 84aa1dc..d82ba55 100644
--- a/api/test/frame_generator_interface.h
+++ b/api/test/frame_generator_interface.h
@@ -48,10 +48,7 @@
   // Change the capture resolution.
   virtual void ChangeResolution(size_t width, size_t height) = 0;
 
-  virtual Resolution GetResolution() const {
-    // TODO(mbonadei): Remove and make it pure virtual.
-    return {.width = 0, .height = 0};
-  }
+  virtual Resolution GetResolution() const = 0;
 };
 
 }  // namespace test