Make SpaceSplitString::set() faster

Make SpaceSplitString::set() faster

The SpaceSplitString in general and SpaceSplitString::set()
in particular are used very intensively in the blink DOM handling code.

This patch obviates an unneeded AtomicString re-creation within SpaceSplitString::set() method, as a result the median time value in the 'modify-element-classname.html' perf test decreased approximately by 2%-2.5%, VTune Amplifier shows the following delta of the total CPU time for the whole test execution: 3.610s - 3.550s = 0.060s 
(Linux desktop x64).

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196374

[Mikhail, 2015-06-02 13:37:23]

mikhail.pozdnyakov@intel.com changed reviewers:
+ mkwst@chromium.org

[Mikhail, 2015-06-02 13:43:21]

PTAL

[Mike West, 2015-06-02 16:24:34]

Wonderful!

LGTM.

[Mikhail, 2015-06-03 06:13:26]

The CQ bit was checked by mikhail.pozdnyakov@intel.com

[commit-bot: I haz the power, 2015-06-03 06:13:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1167693004/1

[commit-bot: I haz the power, 2015-06-03 07:04:27]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196374