content: adds feature for sync continuing when no before-unload handlers

As WebView needs to continue to support the ability to change
navigation from before-unloadhandler callbacks, this feature will
never be enabled for WebView. The rationale for keeping the two
paths is posttask is quite costly at the higher percentiles on android.

BUG=365039


Change-Id: I2b8b22e4f0e1ad1ccb7b6ee5dc5dc3605335257e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3475485
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Charles Reis <creis@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#977402}