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This is a reland of 17bbb4dd

WEB_ENGINE_EXPORT was missing in the original, causing the component
build to fail. It has been added now.

Original change's description:
> [fuchsia] Implement NavigationPolicyThrottle for navigation blocking
>
> * Add an implementation of NavigationPolicyThrottle to WebEngine
> * Add NavigationPolicyHandler, which keeps track of throttles and checks
>   if navigations need to be passed on to the client
> * Add a FakeNavigationPolicyProvider for tests
>
> Test: NavigationPolicyThrottleTest
> Bug: 1117601
> Change-Id: I0bdf94345b10d829bea4aeb85f74204970a97239
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519039
> Commit-Queue: Sharon Yang <yangsharon@chromium.org>
> Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#826625}

Bug: 1117601
Change-Id: If1b5e3e9e6e199795be7076641c73edfdf5fafb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534608
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826935}