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Previously, only PluginPowerSaverBrowserTest had access to the ability
to await the placeholder to be ready.

Now that we want to do this for a PDF placeholder browser test as well,
move this functionality to a test support class.

This is patch 1/3 for this objective.

Bug: 887752, 879149, 878871
Change-Id: Ib0bc70f21d25ec35874025c0952f7c42555fce5b
Reviewed-on: https://chromium-review.googlesource.com/c/1335781
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608399}