Change PrepareErrorPage to be asynchronous

Instead of passing in a pointer to |error_html|, PrepareErrorPage now
takes a callback. This is done in preparation to implement
committed interstitials for SSL errors, since SSL error HTML must be
generated asynchronously to account for asynchronous captive portal
detection.

Bug: 987407
Change-Id: Ia50442072e0ff1515a3d941f0f27cf68f411aae0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742731
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685296}
8 files changed