This change switches corruption_test, which previously used direct file
I/O to corrupt table files for open databases, to use InMemEnv. Using an
Env eliminates some platform dependencies thus simplifying the tests.

Also removed EnvWindowsTestHelper::RelaxFilePermissions().  This was
only added because the Windows Env opens files for exclusive access.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=239305329

This change adds a native Windows port (port_windows.h) and a
Windows Env (WindowsEnv).

Note1: "small" is defined when including <Windows.h> so some
parameters were renamed to avoid conflict.

Note2: leveldb::Env defines the method: "DeleteFile" which is
also a constant defined when including <Windows.h>. The solution
was to ensure this macro is defined in env.h which forces
the function, when compiled, to be either DeleteFileA or
DeleteFileW when building for MBCS or UNICODE respectively.

This resolves #519 on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=236364778