Update V8 to version 4.3.48.

Update V8 to version 4.3.48.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/a2bf336a..c18bc92e

Please follow these instructions for assigning/CC'ing issues:
https://code.google.com/p/v8-wiki/wiki/TriagingIssues

TBR=hablich@chromium.org,machenbach@chromium.org,yangguo@chromium.org,vogelheim@chromium.org

Committed: https://crrev.com/7cab463ee65c5a1546477bc60148de13e1ac863b
Cr-Commit-Position: refs/heads/master@{#321442}

[v8-autoroll, 2015-03-19 20:50:37]

The CQ bit was checked by v8-autoroll@chromium.org

[commit-bot: I haz the power, 2015-03-19 20:51:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1003363004/1

[Michael Achenbach, 2015-03-19 21:36:48]

CC TPM. Potential trouble-maker roll.

[commit-bot: I haz the power, 2015-03-19 21:56:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-19 21:57:40]

Patchset 1 (id:??) landed as
https://crrev.com/7cab463ee65c5a1546477bc60148de13e1ac863b
Cr-Commit-Position: refs/heads/master@{#321442}

[Michael Achenbach, 2015-03-19 22:04:33]

A cherry-pick roll with the risky CL removed is in preparation.

[Michael Achenbach, 2015-03-19 22:28:55]

https://codereview.chromium.org/1019743003/ is the same as this roll with a
revert of a CL suspected for the GC failures.