When we don't know the call count for a given call site (i.e. for
inlined accessors), we put 0 as call frequency so far. But as of
https://codereview.chromium.org/2859433002, this would completely
disable the inlining of those calls, since 0 is interpreted as never
called, which is not what we want. So instead of defaulting to 0,
add a dedicated sentinel, whose value is NaN, which makes the call
site eligible for inlining, but not high priority (as it was before
the CL mentioned above).

BUG=v8:4493,v8:5267
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2856103002
Cr-Commit-Position: refs/heads/master@{#45053}