Implement and wire in NetworkServiceClient::OnFileUploadRequested.

This checks ChildProcessSecurityPolicy before opening the requested
files for upload.

Bug: 845612
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I16538e38bcb0a44acaf8c6dab4342dea8872f81f
Reviewed-on: https://chromium-review.googlesource.com/1098150
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570986}
14 files changed