The default NaN mode was originally used to identify holes in double
arrays. With (https://codereview.chromium.org/863633002/) signalling
NaNs are used for that, and the default NaN mode is not needed anymore.
Without the default NaN mode it is easier to satisfy the WebAssembly
spec which requires that quiet NaNs are preserved.

R=titzer@chromium.org, rodolph.perfetta@arm.com, bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1884193002

Cr-Commit-Position: refs/heads/master@{#35518}