idlharness.js: add_dependency_idls: Include partials in attribute type imports.

Change-Id: I57ad6795e9d53b800c70815ba0ab6808849db611
Reviewed-on: https://chromium-review.googlesource.com/1069663
Commit-Queue: Luke Bj <lukebjerring@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562880}
2 files changed