Bound ppapi process totals when isolating by origin.

Otherwise, we risk having an unbounded number of processes.
Results in a "failed to load plugin" page once N == 15 created.
Add test to fork off a handful.

No plugins are yet isolated by origin, so no observable change
to chrome. That will happen in a subsequent CL.

Follow-up to:
  https://chromium-review.googlesource.com/c/chromium/src/+/915182

Bug: 809614
Change-Id: Ie71d41d03d380e31f006173b659023e3de0e3ac8
Reviewed-on: https://chromium-review.googlesource.com/942235
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540565}
3 files changed