[esnext] implement StringPad spec changes from March TC39 meeting

[esnext] implement StringPad spec changes from March TC39 meeting

Return the empty string from StringPad if the filler argument is the empty
string, effectively returning the string unchanged in StringPadStart and
StringPadEnd.

```
"foo".padStart(100000, "") -> "foo"
```

BUG=v8:4957, v8:4954
LOG=N
R=littledan@chromium.org

Committed: https://crrev.com/ff956f4dee0a7c5b50916e04e02444e46931ce7e
Cr-Commit-Position: refs/heads/master@{#35837}

[caitp (gmail), 2016-04-27 21:33:11]

PTAL for quick fix

[Dan Ehrenberg, 2016-04-27 21:36:28]

lgtm

[caitp (gmail), 2016-04-27 21:42:28]

The CQ bit was checked by caitpotter88@gmail.com

[commit-bot: I haz the power, 2016-04-27 21:42:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1926773003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1926773003/1

[commit-bot: I haz the power, 2016-04-27 22:02:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-05-01 22:01:35]

Description was changed from

==========
[esnext] implement StringPad spec changes from March TC39 meeting

Return the empty string from StringPad if the filler argument is the empty
string, effectively returning the string unchanged in StringPadStart and
StringPadEnd.

```
"foo".padStart(100000, "") -> "foo"
```

BUG=v8:4957, v8:4954
LOG=N
R=littledan@chromium.org
==========

to

==========
[esnext] implement StringPad spec changes from March TC39 meeting

Return the empty string from StringPad if the filler argument is the empty
string, effectively returning the string unchanged in StringPadStart and
StringPadEnd.

```
"foo".padStart(100000, "") -> "foo"
```

BUG=v8:4957, v8:4954
LOG=N
R=littledan@chromium.org

Committed: https://crrev.com/ff956f4dee0a7c5b50916e04e02444e46931ce7e
Cr-Commit-Position: refs/heads/master@{#35837}
==========

[commit-bot: I haz the power, 2016-05-01 22:01:36]

Patchset 1 (id:??) landed as
https://crrev.com/ff956f4dee0a7c5b50916e04e02444e46931ce7e
Cr-Commit-Position: refs/heads/master@{#35837}