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This CL adds a method to ClientTransferCache that allows the client to
get a ClientDiscardableHandle without
ClientTransferCache::Client::IssueCreateTransferCacheEntry getting
called so that the client can essentially start a locked entry but
trigger the creation of the service-side entry using some other
mechanism, e.g., an IPC message.

This is needed for hardware-accelerated image decodes because the decode
will be started by sending an IPC message, and the service-side cache
entry shouldn't be created until the decode is done, so the usual flow
of triggering the cache entry creation using the command buffer won't
do.

Additionally, some documentation is updated.

Bug: 868400
Change-Id: I24cf9b921b8a0ea2e7038c1801f2b7c36fda1927
Reviewed-on: https://chromium-review.googlesource.com/c/1328184Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607438}