Don't preload all tokens

Turns out we're not really ready to do this.
The preloader has lots of bugs at least in the
re-use-existing-preload logic. Yes, we should fix
these bugs. Yes, making all tokens go through the
preloader would force us to fix these bugs. But
until we do, it seems better to sweep them back
under the rug (and just have flakiness in the wild)
rather than making preloader bugs completely
unavoidable by forcing every token through it.

This is a partial revert of:
https://codereview.chromium.org/302063002/

BUG=356292,388308,389226

Review URL: https://codereview.chromium.org/359623004

git-svn-id: svn://svn.chromium.org/blink/trunk@177044 bbb929c8-8fbe-4397-9dbb-9b2b20218538