Avoid excessive data copying for ExternalStreamingStream::SetBookmark.

Avoid excessive data copying for ExternalStreamingStream::SetBookmark.

BUG=v8:4422
R=jochen@chromium.org
LOG=Y

Committed: https://crrev.com/1025d34acf37187d1c872eabe5f603b16ffdaa6a
Cr-Commit-Position: refs/heads/master@{#30763}

[jochen (gone - plz use gerrit), 2015-09-15 13:00:04]

lgtm

[jochen (gone - plz use gerrit), 2015-09-15 13:00:24]

is this enough to regain the entire performance?

[vogelheim, 2015-09-15 13:45:09]

On 2015/09/15 13:00:24, jochen wrote:
> is this enough to regain the entire performance?

Looks like it. It eliminates the 'slow' case that the (original) bug talks
about, and also speeds up the 'fast case'.

I'll do a bit more testing before submitting, though. Mainly for correctness.
I'll also test against disabled bookmarking.

[vogelheim, 2015-09-16 11:35:17]

The CQ bit was checked by vogelheim@chromium.org

[vogelheim, 2015-09-16 11:35:17]

The patchset sent to the CQ was uploaded after l-g-t-m from jochen@chromium.org
Link to the patchset: https://codereview.chromium.org/1346613002/#ps20001
(title: "Make compilers happy (signed/unsigned comparison)")

[commit-bot: I haz the power, 2015-09-16 11:35:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1346613002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1346613002/20001

[commit-bot: I haz the power, 2015-09-16 11:37:07]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-16 11:37:25]

Patchset 2 (id:??) landed as
https://crrev.com/1025d34acf37187d1c872eabe5f603b16ffdaa6a
Cr-Commit-Position: refs/heads/master@{#30763}