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This reverts commit 5ff974fc.

Original CL was https://chromium-review.googlesource.com/953979
This CL differs from the original in that in the test framework,
PDFs are loaded into separate tabs, instead of on top of each
other. Under slow test infrastructure (e.g. MSAN), the plugin
processes would exit before we could complete the loads and
count them. This fixes the test (and the shipping code is unchanged
from the original).

TBR=jam@chromium.org

Bug: 809614
Change-Id: Id13a97023a60d86fb72c47d580c001b737bf17bf
Reviewed-on: https://chromium-review.googlesource.com/959236
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542652}