[V8][Performance] Optimize createTextNode(), createElement(), cloneNode(), etc

[V8][Performance] Optimize createTextNode(), createElement(), cloneNode(), etc
https://bugs.webkit.org/show_bug.cgi?id=82201

Reviewed by Adam Barth.

This patch improves performance of createTextNode() by 13%, createElement() by 14%,
and cloneNode() by 16%. Similar performance improvement will be observed in
DOM methods that create a new object every time.

Performance test: https://bugs.webkit.org/attachment.cgi?id=133799

The performance test results are as follows. Since the performance of V8's GC is
really unstable, the average of measured times makes no sense in Chromium.
Instead, let us focus on the median. I believe that this performance
improvement has impact on Dromaeo, but we cannot observe the improvement
due to the unsteadiness of V8's GC, as shown below.

Chromium/V8/Linux (without the patch):
createTextNode : median=277ms (mean=460.88ms, min=270ms, max=3381ms)
createElement : median=379ms (mean=637.52ms, min=372ms, max=3022ms)
cloneNode : median=369ms (mean=581.72ms, min=363ms, max=3050ms)
Dromaeo/dom-modify/createElement: 439.17runs/s +-31.60% (<--- pretty noisy)
Dromaeo/dom-modify/createTextNode: 287.71runs/s +-28.39% (<--- pretty noisy)
Dromaeo/dom-modify/cloneNode: 174.62runs/s +-25.68% (<--- pretty noisy)

Chromium/V8/Linux (with the patch):
createTextNode : median=240ms (mean=411.12ms, min=237ms, max=2965ms)
createElement : median=325ms (mean=585.30ms, min=317ms, max=2984ms)
cloneNode : median=310ms (mean=522.48ms, min=302ms, max=2988ms)
Dromaeo/dom-modify/createElement: 507.15runs/s +-36.00% (<--- pretty noisy)
Dromaeo/dom-modify/createTextNode: 251.01runs/s +-6.57%
Dromaeo/dom-modify/cloneNode: 177.85runs/s +-28.74% (<--- pretty noisy)

Chromium/V8/Mac (without the patch):
createTextNode : median=317ms (mean=439.08ms, min=303ms, max=3126ms)
createElement : median=403ms (mean=695.70ms, min=398ms, max=5615ms)
cloneNode : median=384ms (mean=577.96ms, min=372ms, max=5313ms)
Dromaeo/dom-modify/createElement: 493.89runs/s +-28.32% (<--- pretty noisy)
Dromaeo/dom-modify/createTextNode: 279.66runs/s +-1.91%
Dromaeo/dom-modify/cloneNode: 173.06runs/s +-24.41% (<--- pretty noisy)

Chromium/V8/Mac (with the patch):
createTextNode : median=277ms (mean=460.88ms, min=270ms, max=3381ms)
createElement : median=379ms (mean=637.52ms, min=372ms, max=3022ms)
cloneNode : median=369ms (mean=581.72ms, min=363ms, max=3050ms)
Dromaeo/dom-modify/createElement: 510.47runs/s +-28.13% (<--- pretty noisy)
Dromaeo/dom-modify/createTextNode: 215.80runs/s +-20.99% (<--- pretty noisy)
Dromaeo/dom-modify/cloneNode: 174.41runs/s +-24.85% (<--- pretty noisy)

Safari/JavaScriptCore/Mac:
createTextNode : median=142ms (mean=141.04ms, min=110ms, max=168ms)
createElement : median=234ms (mean=245.74ms, min=219ms, max=305ms)
cloneNode : median=210ms (mean=213.36ms, min=204ms, max=284ms)
Dromaeo/dom-modify/createElement: 822.49runs/s +-1.69%
Dromaeo/dom-modify/createTextNode: 735.57runs/s +-0.91%
Dromaeo/dom-modify/cloneNode: 135.20runs/s +-4.13%

This patch makes the following two optimizations:

[1] If the currently running context is equal to the context that we are about to enter,
we do not call context->Enter().
[2] We do not create a Local handle of the context until we really need to enter the context.

* bindings/scripts/CodeGeneratorV8.pm:
(GenerateToV8Converters):
* bindings/v8/V8Proxy.cpp:
(WebCore::V8Proxy::persistentContext):
(WebCore):
* bindings/v8/V8Proxy.h:
(V8Proxy):

* WebCore/bindings/scripts/test/V8/V8TestActiveDOMObject.cpp: Updated run-bindings-tests results.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@112218 268f45cc-cd09-0410-ab3c-d52691b4dbfc