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This change handles the cases of brand new IDL files added to a build,
or cases where files under third_party\win_build_output\midl are
(unexpectedly) missing. This change also has some minor refactoring of
the |writes_tlb| midl.gni option.

Bug: 1149764
Change-Id: If2e608f2e986bd3fbc13a3a7b699801660c2ffa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2543367
Commit-Queue: S. Ganesh <ganesh@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828319}