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When we migrated GpuMemoryBufferHandle to the new shared memory API
we didn't take into account that the previous SharedMemoryHandle would
not enforce the size of the shared memory region stayed the same,
while UnsafeSharedMemoryRegion does.

Before using UnsafeSharedMemoryRegion we could pass the handle,
ignore shm_pool_resize and map to an offset that is out of the original
size we created the pool with.
This breaks with UnsafeSharedMemoryRegion that checks the region we're
mapping is contained in size.

This CL implements shm_pool_resize, adding protocol checks and recreating
UnsafeSharedMemoryRegion with the proper new size.

Bug: 961155
Change-Id: I443feca90ad332c40cb74f8a76182a20a565d7a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1608800
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660082}