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This corrects the expectations of
fast/canvas/OffscreenCanvas-placeholder-image-source.html to align with
reality. There does not appear to be any kind of guarantee that the
placeholder canvas will be updated within the two scheduler iterations
the test was designed to wait for.

Instead the test now keeps checking the placeholder canvas until it's
updated, which still happens very quickly in practice. If something
breaks, the test will timeout.

Bug: 872076
Change-Id: Idd1a3018e63f5ad2df540d642f5abb17b80410d5
Reviewed-on: https://chromium-review.googlesource.com/c/1292717Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#601591}