Ignore certain messages in plugin broker process if they are not sent by the browser.

BUG=610600
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2069853002
Cr-Commit-Position: refs/heads/master@{#400495}
(cherry picked from commit 92cad45212731b59e74c8f2a2913691db3e9d770)

Review URL: https://codereview.chromium.org/2122313004 .

Cr-Commit-Position: refs/branch-heads/2743@{#588}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}
9 files changed