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Reports the PatchLevel of the current running Windows OS as
reported by a call to VerQueryValue() on kernel32.dll.

Also, add comments for missing functions in windows_version.h
and add Kernel32VersionNumber function on OSInfo for
determining Kernel32 version.

BUG=1130738

Change-Id: I70c652e6524e15382a511cfc12311d5ffffedb1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444452Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813824}