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To support unbundled updates we need to stop depending on system
libraries. Disable using the system version of libexpat and instead link
Chromium's copy, and stop removing the expat sources from the checkout.

This increases binary size by ~64KB.

Also remove an obsolete comment about using the system version of
sqlite; this was never implemented and now will not be.

BUG=409851

Review URL: https://codereview.chromium.org/534603002

Cr-Commit-Position: refs/heads/master@{#293115}