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Removes the ability to set a prerender speculation mode for custom tabs,
making the background (hidden) tab the only option. Where appropriate,
method names mentioning "prerender" have been changed to generically
refer to "speculation". In one case shims for
shouldPrerenderOnCellularForSession, etc, have been left in so that
downstream code can be changed in a future CL.

This is a reland of the CL after a downstream patch that allow removing
the constants in SpeculationParams.

TBR: lizeb@chromium.org,pasko@chromium.org,bauerb@chromium.org,gayane@chromium.org
Bug: 755921,710720, 828968
Change-Id: I9a8002d1f0b8c2513d32469d2d7c4ed787912cb4
Reviewed-on: https://chromium-review.googlesource.com/998100
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Reviewed-by: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550116}