Force enable HTML Imports for HeadlessProtocolBrowserTest temporally.

This patch is a preparation for Shadow DOM V0, Custom Element V0 (CEv0) and HTML Imports removal.
Since we soon make these APIs only available for test by turning the json5 feature flag
from status: "stable" to status: "test", this patch is a part of it.
(you can see exact flag turning in my WIP patch:
 https://chromium-review.googlesource.com/c/chromium/src/+/1399774)

We should remove/migration the tests using the old APIs.

Bug: 911943
Change-Id: I188c08951c43b7ce0168df17a26ab1805c69ddb8
Reviewed-on: https://chromium-review.googlesource.com/c/1404767
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621868}
1 file changed