Make RegExp use ToLength on lastIndex when flag is turned on

Make RegExp use ToLength on lastIndex when flag is turned on

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

Committed: https://crrev.com/5c1ef6ac823e2a57805d3eb8126f796dfe479aed
Cr-Commit-Position: refs/heads/master@{#31306}

[Dan Ehrenberg, 2015-10-15 13:03:36]

The CQ bit was checked by littledan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-15 13:03:45]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1394023005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1394023005/1

[adamk, 2015-10-15 13:09:10]

lgtm

[Benedikt Meurer, 2015-10-15 13:31:06]

bmeurer@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Benedikt Meurer, 2015-10-15 13:31:07]

LGTM

[commit-bot: I haz the power, 2015-10-15 13:52:07]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-15 13:52:08]

Dry run: This issue passed the CQ dry run.

[Dan Ehrenberg, 2015-10-15 14:04:51]

The CQ bit was checked by littledan@chromium.org

[commit-bot: I haz the power, 2015-10-15 14:05:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1394023005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1394023005/1

[commit-bot: I haz the power, 2015-10-15 14:10:41]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-15 14:10:56]

Patchset 1 (id:??) landed as
https://crrev.com/5c1ef6ac823e2a57805d3eb8126f796dfe479aed
Cr-Commit-Position: refs/heads/master@{#31306}