When the input to Number.parseInt is a HeapNumber in Signed32 range, we
can just return the (truncated) input value (i.e. we need to map -0 to
0 due to the ToString conversion).

R=jarin@chromium.org

Review-Url: https://chromiumcodereview.appspot.com/2432923002
Cr-Commit-Position: refs/heads/master@{#40419}