[MappableSI] Remove GpuMemoryBufferManager usage from

Remove use of SharedGpuContext::GetGpuMemoryBufferManager() from
CanvasResourceProvider since CanavsResourceProvider have been converted
to use MappableSI.

SharedGpuContext::GetGpuMemoryBufferManager() check was redundant in the
first place as it is always present and is ClientGpuMemoryBufferManager
for production code or TestGpuMemoryBufferManager for tests. So it is
safe to remove that check irrespective of MappableSI.

Additionally, with MappableSI, the duty of GpuMemoryBufferManager is
performed by SharedImageInterface and relying on GpuMemoryBufferManager
is no longer required with MappableSI.

Bug: 404905696
Change-Id: I9da4dbf2ce371bdb4d7010d6f1a387a6ef04bc04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6374347
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: vikas soni <vikassoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438837}