Prerender: Override user agent for prerendering on WebView

According to [1], navigation on WebView always enables user agent
override, but prerendering navigation doesn't set an override option and
fails DCHECK on activation due to the parameter mismatch  (see the
associated issue).

This CL allows AwWebContentsDelegate to set the option. Android Chrome
with the desktop mode has a similar issue (https://crbug.com/1406149)
but this CL only fixes the WebView case for now.

Note that prerendering on WebView is an experimental feature that is
still behind the feature flag (kWebViewPrerender2).

[1]https://source.chromium.org/chromium/chromium/src/+/main:android_webview/java/src/org/chromium/android_webview/AwContents.java;l=2564-2566;drc=3cc22c22d7637bc5604e8fef3b0882b51a762901

Bug: 1518322
Change-Id: I58359cd8e83917f6070f47ec1e9a6932ebb7cfb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5216286
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1249970}