WebUI: Add PromiseResolver#isFulfilled to avoid double resolution/rejection

R=dpapad@chromium.org
BUG=855259

Change-Id: I42160189ba60c849bcb9cd9578f5e27c47c02791
Reviewed-on: https://chromium-review.googlesource.com/c/1364693
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614561}
2 files changed