WebSQL: Simplify API for DatabaseTracker::DeleteDataForOrigin.

The method currently follows an unfortunate maybe-async pattern
established in //net whereby operations may either report an outcome
synchronously by returning a net::Error other than net::ERR_IO_PENDING,
or report the outcome asynchronously by invoking a callback.

This CL changes the method to unconditionally run the callback with the
operation outcome. This is intended to simplify the mental model and
control flow around the method.

Change-Id: I4337cb5e93820596510b02c0b6bde75a6f3111f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632853
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844996}