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Chrome used the default SQLite configuration where databases were first
opened in the NORMAL (not exclusive) locking mode, and then databases
that opt in were switched to the EXCLUSIVE locking mode via a "PRAGMA
locking_mode" statement.

This CL switches the default. Databases are first opened in EXCLUSIVE
mode by changing the SQLITE_DEFAULT_LOCKING_MODE build configuration
macro. Databases that don't opt into exclusive mode are switched to the
NORMAL locking mode via a "PRAGMA locking_mode" statement.

This CL should not introduce any behavior changes in most cases. The
result of the "PRAGMA locking_mode" statement was previously ignored,
and is now checked. So, if the desired locking mode cannot be set,
sql::Database::Open() will now fail.

In the long run, Chrome's SQLite usage should be migrated to use the
exclusive locking mode, for the benefits outlined in
https://crbug.com/1120969. When that happens, the "PRAGMA locking_mode"
can go away.

Bug: 56559, 1120969
Change-Id: Ia9ac996fae45890bb5ca11168422e15e594087ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460982
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815540}