Disable cross-process navigations in --single-process.

This flag takes precedence over --site-per-process and other
site isolation modes.

BUG=688617
TEST=Cross-site subframes work with --single-process --site-per-process
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2964033002
Cr-Commit-Position: refs/heads/master@{#485292}
4 files changed