PlzNavigate: support for srcdoc navigations

This CL introduces support for srcdoc navigations. They are no longer sent to
the browser, as they make no netwrok request. On commit, the created
NavigationHandle is also made properly aware that the navigation was an srcdoc
navigation.

BUG=576263
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1691333002

Cr-Commit-Position: refs/heads/master@{#375456}
6 files changed