[//gpu] Decouple GMBSharedMemoryImpl::{AllocateForTesting, CreateGMB}

We cannot remove AllocateForTesting() in the immediate as it's broadly
used within gpu_memory_buffer_test_template.h. However, we're looking
to remove GMBSharedMemoryImpl::CreateGpuMemoryBuffer() [1]. This CL
decouples the implementations of the two methods. We'll loop back around
to gpu_memory_buffer_test_template.h as part of decomposing
GpuMemoryBufferImpl.

[1] https://source.chromium.org/chromium/chromium/src/+/main:gpu/ipc/common/gpu_memory_buffer_impl_shared_memory.cc;l=66-85?q=GpuMemoryBufferImplSharedMemory::CreateGp&ss=chromium

Bug: 404905709
Change-Id: I00d2c39d1851bfdbfeae1b63c2d5512fe6a449f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6772972
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Antonio Sartori <antoniosartori@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1489965}