[Fuchsia] Disable GpuMemoryBuffer unittests on Fuchsia

On Fuchsia sysmem-based NativePixmap implementationd depends on Vulkan,
but we don't have Vulkan support in the test environment yet, so there
is no way to run these tests at the moment.

Bug: 957700
Change-Id: Ic55206d3fd5d94bd0198f7308843a1a36b936788
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588200
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655324}