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This is a cleanup and doesn't change functionalities.

Before this change, each browser test case had its own html file that
triggers <link rel=prerender>. This was not scalable. After this change,
browser tests share the common html file that can dynamically add
<link rel=prerender> from C++.

In addition to that, this CL factors out common routines into helper
functions.

Bug: 1132746
Change-Id: I7aea316da481c86832314384f2d2d997c30deaea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567054
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832668}