[cc/tiles] Fix transfer cache entry deletion in GpuImageDecodeCache

The renderer-side GpuImageDecodeCache is referencing GPU process side
memory in the transfer cache. When memory is deleted from it, the
deletion can actually happen much later, since the delete command may
sit in a buffer. A previous commit
(https://chromium-review.googlesource.com/c/chromium/src/+/4704673)
added a "flush" call to make sure that the deletion makes its way to the
GPU process even when no outside event forces it (e.g. a frame commit,
or other memory reclaim action). However, it doesn't work, because it
only flushes "deferred messages" (see GpuChannelHost::InternalFlush()),
but transfer cache entry deletion is a regular command buffer command
(see DeleteTransferCacheEntryINTERNAL()).

This means that we need a shallow command buffer flush to ensure that
deletion commands are actually processed. This has been a longstanding
bug in (at least) GpuImageDecodeCache, impacting
GpuImageDecodeCache::ClearCache() (and its callers), in particular
memory pressure signals processing.

Locally, loading a static page with lots of images, we observe that
prior to this commit, >200MiB can be used by the transfer cache, and not
go away as planned until a new frame is produced. On an ARM64 mac
laptop, this is verified with IOAccelerator memory, which is backing
transfer cache memory GPU-side.

$ vmmap $GPU_PROCESS_PID
[...]
                                VIRTUAL RESIDENT    DIRTY  SWAPPED VOLATILE   NONVOL    EMPTY   REGION
REGION TYPE                        SIZE     SIZE     SIZE     SIZE     SIZE     SIZE     SIZE    COUNT (non-coalesced)
===========                     ======= ========    =====  ======= ========   ======    =====  =======
[...]
IOAccelerator                     22.4M     112K     112K    6880K       0K       0K    15.4M      198

Bug: 1441409
Change-Id: Ib49eb8283ac969fb219a3524ff54ce4715897f3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4859345
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1195900}