This switches the source positions consumed by {WasmFunctionBuilder} to
have type {size_t} instead of {int}. All checks for accidental loss due
to overflow in static casts can now be moved into one place.

R=clemensh@chromium.org

Change-Id: Id0b0da5fe799eac794c9d610c4c4c9a72a587b3f
Reviewed-on: https://chromium-review.googlesource.com/857196Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50449}