[cleanup] Add missing file to BUILD.gn
[cleanup] Add missing file to BUILD.gn

A previous change https://crrev.com/c/2120014 introduces a unittest
file, but did not add it to the corresponding BUILD.gn.

This CL adds it thereby enabling the two unittests in that file.

Change-Id: I8a2acfa00c8a22cd10de7e69b64394a63ffc4b1c
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2172428
Commit-Queue: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Commit-Queue: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Auto-Submit: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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