Also migrate the Number constructor to a native builtin, using the
same mechanism already used by the String constructor. Otherwise just
parsing and compiling the Number constructor to optimized code already
eats 2ms on desktop for no good reason, and the resulting optimized
code is not even close to awesome.

Drive-by-fix: Use correct context for the [[Construct]] case of the
String constructor as well, and share some code with it.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1573243009

Cr-Commit-Position: refs/heads/master@{#33265}