Prerender: Add basic browser tests for Prerender2

This CL adds basic browser tests for <link rel=prerender> using
Prerender2.

Bug: 1132746
Change-Id: I260fb9652839ee30327f5bfd44096ea570aceb9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489061


Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820064}