Remove g_NaN global variable and its constructor.
Remove g_NaN global variable and its constructor.

Doing the type conversion on demand is just as efficient as doing it at
startup time, and makes for more efficient startup.

Also mark g_nan as const, to reduce .data section size and enforce
desired semantics.

BUG=441899
R=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/788143009

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