Migrate Date.prototype.valueOf and Date.prototype[ @@toPrimitive ]
to use the CodeStubAssembler, to avoid going through C++ always
when comparing or subtracting two dates.

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2608143003
Cr-Commit-Position: refs/heads/master@{#42030}