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Previously SysmemBufferCollection could crash if it failed to initialize
(e.g. due to Allocator interface missing).

Change-Id: I72f83ebe7ad858ab908ad42fb08aeff895327c67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614001
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660928}