I asked myself the same question.
With PlzNavigate, the InterceptNavigationDelegateImpl is only used to post an onPageStarted event. If we want, we probably can remove the InterceptNavigationDelegateImpl and post onPageStarted just after calling ShouldOverrideUrlLoading in NavigationRequest::BeginNavigation().

There are a lot WebView tests that will fail if you don't post onPageStarted.

onPageStarted and onPageFinished are emitted only if shouldOverrideUrlLoading() return false for the initial navigation. It doesn't matter if shouldOverrideUrlLoading() returns false after a redirect.

FYI: I tried to move the WebView code out of content/ and put it in android_webview/
https://chromium-review.googlesource.com/c/chromium/src/+/697811.
It uses a NavigationThrottle. It could be used to post the onPageStarted event as well. But I had some issues with the onPageFinished one.

Right, I'm a bit confused when it comes to the case where PlzNavigate is turned on - in that case we will always post onPageStarted from InterceptNavigationDelegateImpl, while with the old code path we would only post onPageStarted if shouldOverrideUrlLoading returns false. Isn't that an unwanted behaviour change?

With PlzNavigate, if ShouldOverrideUrlLoading return true in NavigationRequest::BeginNavigation(), we don't create a navigation (The navigation is represented by the NavigationHandle)
It means that the InterceptNavigationThrottle is not created. So it can't post a onPageStarted event.

The behavior is the same with/without PlzNavigate AFAIK

Ah ok, cool, would it be possible to add a comment about this somewhere for mere confused mortals like me? ;)

Yes, I was confused too when I was trying to fix issues with ShouldOverrideUrlLoading, a comment would be nice. I will add it if I make or see a patch about this soon or in a separate CL then.

Great, thanks!