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It took me a while to figure out what was wrong with my valid-looking
new IDL file, this diagnostic would have saved me some time. However,
I don't know if it's potentially confusing for other errors people
might make, or if there are better ways to address this.

Bug: 1041101
Change-Id: Ifae76b7d77e558f1888be0b70ae52fd53fc7514a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995712Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733039}