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We previously only registered share targets that received files.

Test uses web_share_target/poster.html, a web app added in
https://chromium-review.googlesource.com/c/chromium/src/+/2584625


Bug: 1125880
Change-Id: Iee54e946a26128b72c2c86df47e58768ca917c63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2584430
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836913}