GetAppropriateMmapSize() is called to validate the database as
error-free before enabling memory-mapped mode.  The validation status is
stored in the [meta] table, since new databases have no [meta] table it
previously defaulted to not turning on memory-mapping.  Then on second
open the file would be validated.

This change uses the lack of [meta] table as a signal that the file is
new and should be safe to memory-map.  Additionally the code creating
[meta] pre-populates that setting.

Additionally disable memory-mapped mode for databases which do not make
use of the [meta] table.  This makes the existing settings explicit.

BUG=537742, 555578
TBR=rdevlin.cronin@chromium.org, michaeln@chromium.org, zea@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/1533703002

Cr-Commit-Position: refs/heads/master@{#365959}