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Instead of reading PreRead options from the registry every time they
are needed, read them once and store them in a global variable.
This optimization isn't immediately useful as PreRead options are
currently read only once per process. However, with the upcoming
/prefetch:# experiment, the browser process will need the PreRead
options every time it launches a process.

This CL also has these benefits:
- The caller of GetPreReadOptions doesn't need to know the product
  registry path. This will allow content to get the PreRead options
  to decide whether it should use a /prefetch:# switch when
  launching a process.
- The PreRead options are returned as a bit field instead of by
  setting multiple booleans. This will facilitate the addition of
  new PreRead options.

BUG=577698

Review URL: https://codereview.chromium.org/1610733002

Cr-Commit-Position: refs/heads/master@{#370805}