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Currently, we never check for deleted entries until the client tries
to lock an entry. In some cases (transfer cache), the client may never
delete or re-lock an entry after creating it, relying on the service
side to purge it. In these cases the client should eventually pick up
these deletions allowing handles to be re-used.

To handle this, we check for deleted entries any time we are about to
allocate a new buffer.

R=khushalsagar

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ib4edbf7e339f213cff7a7df85771ecfeb5113936
Reviewed-on: https://chromium-review.googlesource.com/1147495
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577364}