We cannot rely on base::OS::GetCurrentProcessId() as it is sandboxed.

BUG=chromium:823874

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I6aaa7f7f7dacbc8aba43581f9d489e4cc47799ff
Reviewed-on: https://chromium-review.googlesource.com/985425Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52303}