Re-add resource speculative prefetching code

Re-add resource speculative prefetching code

This CL fixes the memory leak bug in CL 462423004.
See https://codereview.chromium.org/462423004/ for previous code reviews.

-------------------------
 Original CL description
-------------------------

Revert CL 117933003. Re-add speculative resource prefetching code.
https://codereview.chromium.org/117933003/

The speculative resource prefetching code was experimental code developed by
shishir@. He found that it has little improvement on desktop Chrome (win). We
though this should be beneficial to mobile browsers. After discussing with
tburkard@ and kenjibaheux@, we decided to bring the code back and do more
analysis on mobile devices. Reverting the patchset to re-add the code is the
first step.

The following design doc has discussed all related approaches and action items.
https://docs.google.com/a/google.com/document/d/1ie3hu-zNNXvmTXm3aJAtKUGOh6nZfbNjA0aZE1bzzIg/edit?usp=sharing

BUG=408399, 405690

Review URL: https://codereview.chromium.org/576313002

Cr-Commit-Position: refs/heads/master@{#295626}