sql: Replace corruption in SQLDatabaseTest.FullIntegrityCheck.

In SQLite 3.38.0, "PRAGMA integrity_check" recovers from an incorrect
page count in the database header, without recording an error. This is
consistent with
https://www.sqlite.org/fileformat2.html#in_header_database_size which
says that SQLite may fall back to the file size, instead of relying on
the database size in the header.

This CL switches the SQLDatabaseTest.FullIntegrityCheck test from
sql::test::CorruptSizeInHeader() to sql::test::CorruptIndexRootPage().
This allows the test to pass in both our current SQLite version
(3.37.1) and in the upcoming SQLite 3.38.0.

This CL also updates the FullIntegrityCheck() documentation, adding API
guarantees in case the database is corrupted.

Bug: 1300442
Change-Id: I70d373a4156fd710f96a50125affe439e7cb5780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3489937
Reviewed-by: Austin Sullivan <asully@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975408}