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It seems that unlike EvalJs, ExecJs doesn't wait for promises to resolve
before returning. This fixes existing portal browsertests to use EvalJs
when waiting for a promise.

Change-Id: Ib1cdb50e79b19fd8296f6b29fd8fbf9931c92ccd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976193Reviewed-by: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726509}