NavigationBodyLoader: allow SetDefersLoading before Start

This scenario can trigger with a nested message loop. We should
not actually try to read the body if it has not been received yet.

Bug: 855189
Change-Id: Idbe0b10dc868302083dcb173f9d79c5d719a61e7
Reviewed-on: https://chromium-review.googlesource.com/c/1416137
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623662}
2 files changed