In the ES2015 spec, RegExp uses ToLength, not ToInteger, on lastIndex
to coerce it to an integer. This patch switches to ToLength when
the --harmony-tolength flag is on, and adds some tests to verify the
new behavior.

BUG=v8:4244
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1394023005

Cr-Commit-Position: refs/heads/master@{#31306}