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It is easy to misuse this class and either:
1. Ignore the return value for |Initialize|
2. Forget to check |IsValid| after initialization

This CL makes it impossible to ignore initialize, so callers must
handler mmap failures.

Bug: None
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: Idc39565ccd80234ed54aafdd6174d37fff259e9f
Reviewed-on: https://chromium-review.googlesource.com/c/1262758Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597221}