ES2015 made a change vs ES5, where the "lastIndex" property of a
RegExp (which can be modified by a user to start the next search at
a different location) is cast to an integer with ToLength rather
than ToInteger. The main difference is on negative numbers, and
this is tested by test262. This patch implements that change on
RegExps and enables the test262 test now that it passes.

R=adamk
LOG=Y
BUG=v8:4244

Review URL: https://codereview.chromium.org/1241713004

Cr-Commit-Position: refs/heads/master@{#29715}