Do not create proxy hosts in the subtree of navigating frame.

When a frame is navigating cross-process, its existing document will be completely destroyed and all child frames will be gone. This means that we don't need to create proxy objects for the new SiteInstance in the subtree of the navigating frame.

BUG=357747

Review URL: https://codereview.chromium.org/536143002

Cr-Commit-Position: refs/heads/master@{#294520}
7 files changed