Optimize WebCore::propertyNameMatchesAttributeName() function

The Intel VTune Amplifier profiler detects
WebCore::propertyNameMatchesAttributeName()
to be the biggest hotspot when running the
'Bindings/named-property-enumerator.html'
performance test. One of the reasons is a lot
of 'WTF::StringImpl::operator[]' invocations
(each invocation checks 'is8Bit()' property).

This patch changes the function's logic so that
'is8Bit()' is checked only once per each argument.

After the patch is applied the profiler is
showing 0.6s of CPU time gain.

Comparison of the test own results looks even more
promising :)

Before:
avg 59.55333689995765 runs/s
median 59.804160976424754 runs/s
stdev 0.5253717029964368 runs/s
min 58.08955085158755 runs/s
max 60.050394290734246 runs/s

After:
avg 90.88615782092175 runs/s
median 91.84269953299489 runs/s
stdev 3.5291535486947656 runs/s
min 82.45819663901538 runs/s
max 95.0907046462235 runs/s

Performance gain for the average value is more than 50%
(Linux desktop 64x, GCC 4.6.3).

Review URL: https://codereview.chromium.org/175373002

git-svn-id: svn://svn.chromium.org/blink/trunk@167609 bbb929c8-8fbe-4397-9dbb-9b2b20218538