Returning a double from ToFloat64 could lead to problems. If value_ has the bit
representation of a signaling NaN (sNaN), then returning it as double can cause
the signaling bit to flip, and value_ is returned as a quiet NaN (qNaN).

The usage of the Double wrapper also, makes the function ToFloat64AsInt
redundant, since the Double wrapper already has the AsUint64() method,
which returns an uint64_t.

R=ahaas@chromium.org

Change-Id: I1e627b97b2fb6110fc702fe58f2b83eb343e9ca2
Reviewed-on: https://chromium-review.googlesource.com/563215
Commit-Queue: Enrico Bacis <enricobacis@google.com>
Reviewed-by: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46680}