With regular I/O, filesystem corruption would cause SQLite to return
SQLITE_IOERR codes. With memory-mapping, filesystem errors can cause a
crash when accessed. There are databases with filesystem corruption,
this CL only enables memory-mapped I/O for parts of the database which
have been successfully read at some point using regular I/O.
BUG=537742
Review URL: https://codereview.chromium.org/1426743006
Cr-Commit-Position: refs/heads/master@{#358672}