Defines a new --harmony-tolength flag, and a ToLengthFlagged() runtime function,
that is used where ES6 requires ToLength(), but a pre-ES6 conversion existed
before. When the flag is disabled, the function uses TO_UINT32(), which is
the pre-ES6 behaviour. When the flag enabled, the ES6-compliant ToLength()
conversion is used.
Based on a patch initially from Diego Pino <dpino@igalia.com>
BUG=v8:3087
LOG=Y
Review URL: https://codereview.chromium.org/1309243003
Cr-Commit-Position: refs/heads/master@{#30772}
