Set WebGPU usage in SI for GpuMemoryBufferVideoFramePool on Mac

A Dawn representation of the SI MediaGmbVideoFramePoolMappableSI can
be created for zero-copy import via WebGPU. This should be reflected
in the SharedImageUsage specified for the SI.

This should have no practical effect other than avoiding a
DumpWithoutCrashing when producing the Dawn representation:
- On Mac, SharedImageCapabilities::supports_scanout_shared_images
  is always true. As a result, GpuMemoryBufferVideoFrameBool always
  requests SCANOUT usage for the SharedImage.
- The only SharedImageBacking that supports SCANOUT on Mac is the
  IOSurfaceImageBacking, which also supports WEBGPU_READ usage.

Bug: 368085619
Change-Id: Ide3e7c5205be814a88481126282adb1589a7aa19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5963741
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1374272}

media/video/gpu_memory_buffer_video_frame_pool.cc

@@ -1229,7 +1229,7 @@ GpuMemoryBufferVideoFramePool::PoolImpl::GetOrCreateFrameResource(
       si_usage |= gpu::SHARED_IMAGE_USAGE_SCANOUT;
     }
 
-#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_MAC)
+#if BUILDFLAG(IS_LINUX) || BUILDFLAG(IS_CHROMEOS)
     // TODO(crbug.com/40194712): Always add the flag once the
     // OzoneImageBacking is by default turned on.
     if (base::CommandLine::ForCurrentProcess()->HasSwitch(
@@ -1237,6 +1237,9 @@ GpuMemoryBufferVideoFramePool::PoolImpl::GetOrCreateFrameResource(
       // This SharedImage may be used for zero-copy import into WebGPU.
       si_usage |= gpu::SHARED_IMAGE_USAGE_WEBGPU_READ;
     }
+#elif BUILDFLAG(IS_MAC)
+    // This SharedImage may be used for zero-copy import into WebGPU.
+    si_usage |= gpu::SHARED_IMAGE_USAGE_WEBGPU_READ;
 #endif
     // Create a Mappable shared image.
     frame_resource->shared_image =