Stop doing async navigation interception on Android Webview

This feature improves page load time a lot on Chrome for Android,
but seems to regress performance on Android Webview [1].

On Webview, it seems like the JNI calls tend to be quite a bit
faster than in Chrome, so the latency of doing an async post task
probably ends up being a detriment.

Previously, this check was done via a platform ifdef. This CL turns
the "async policy" into an enum which is set in the intercept throttle's
constructor.

[1]: https://uma.googleplex.com/p/chrome/variations/?sid=662865f2d88a2d6cbdf5748f0124c8f9

Change-Id: I36ce7dfd21ae3c50f0a2b0568a3f681ebad8f2bf
Reviewed-on: https://chromium-review.googlesource.com/c/1466866
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631869}
10 files changed