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This re-adds the previously approved and committed browsertest, but this
time with a fix for the flakiness.

The body of the test remains the same, but with an extra call to:
PluginTestUtils::WaitForPlaceholderReady(GetActiveWebContents(),
                                         "pdf_embed");

There is also an update the command line flags and an extra include.

This is patch 3/3 for this objective.

Bug: 887752, 879149, 878871
Change-Id: Ie8c70d68fb3e3ebf9e70e5d9f2f1aa2f37b190f4
Reviewed-on: https://chromium-review.googlesource.com/c/1336086Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608411}