Eliminate nullable GpuFenceHandle in Mojo

gfx::GpuFenceHandle already has a notion of a null handle on all
supported platforms. Hence, we don't need to have the corresponding mojo
fields also be nullable.

Bug: 1131616
Change-Id: Ie31775da704aefd26da41516455d397b67c67a27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485629Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#819121}

ui/gfx/mojom/gpu_fence_handle.mojom

@@ -5,11 +5,10 @@
 module gfx.mojom;
 
 // See ui/gfx/ipc/gpu_fence_handle.h
-// A null handle means there is no GPU fence for synchronization.
 struct GpuFenceHandle {
   [EnableIf=is_posix]
-  handle<platform>? native_fd;
+  handle<platform> native_fd;
 
   [EnableIf=is_win]
-  handle<platform>? native_handle;
+  handle<platform> native_handle;
 };