[runtime] Cleanup native methods creation in js/string.js.

This CL replaces usages of utils.InstallFunctions and utils.InstallGetter()
with the DEFINE_METHOD* macros that ensure that the native function is
created in proper form from the beginning. Thus the function will not
require further reconfiguring like adding a computed name or removing of
'prototype' property.

This CL is one of a series of cleanup CL which are the preliminary steps for
improving function closures creation.

Bug: v8:6459
Change-Id: I2a858625242d9b20ae9d7d933d5ba4d6163769f6
Reviewed-on: https://chromium-review.googlesource.com/548076
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46296}