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This makes the externs util code easier to use by other presubmit
tests by reducing a lot of boilerplate.

This also enforces a rule that was being used in practice: the input
& output names have to match (other than file extension).  So instead
of letting the code map "downloads.idl" to the externs "whatever.js"
name, the output must now always be "downloads.js".

If we ignore the Chromecast & app_shell forks of the Chromium project,
this rule holds.  Those forks cause problems by providing extensions
to a few APIs and thus use duplicate source names.  imo, those forks
should be providing their own separate externs tree if they want to
provide externs (which they don't currently), although I suspect the
users are so few that no one will bother.

Bug: 469920
Change-Id: Icb9d2ba4ad78c3c607a09402c82841fb745b4b78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215901
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771917}