Add mechanism to count PDF PPAPI processes in browser tests

Call it as appropriate in pdf_extension_test.cc

Split off from https://chromium-review.googlesource.com/c/chromium/src/+/922727
so as to get a baseline before making further changes.

Bug: 809614
Change-Id: I85e947230fd4f87547dac169e2271c2510c41b29
Reviewed-on: https://chromium-review.googlesource.com/953712
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541866}
5 files changed