Prevent WindowClient.navigate() from cancelling a browser-initiated navigation.

Otherwise, a service worker can prevent you from navigating where you
want to go via the omnibox.

Note: this is similar to WebContentsImpl::OnGoToEntryAtOffset() for
renderer-initiated history navigations.

Bug: 930154
Change-Id: I3a687ccc8ba4420d2369adb24f63c2702bdeeff1
Reviewed-on: https://chromium-review.googlesource.com/c/1477454
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633231}