Make MutableStylePropertySet::findPropertyIndex faster

The Intel VTune Amplifier profile says MutableStylePropertySet::findPropertyIndex
is a biggest hotspot when running 'CSSPropertySetterGetterMethods.html'
performance test.
This patch makes MutableStylePropertySet::findPropertyIndex a bit faster
by making it not to call WTF::Vector::at() in the cycle (WTF::Vector::at()
contains RELEASE_ASSERT affecting the performance).

CPU time diff 17.011s - 16.431s = 0.580s (linux desktop x64, GCC 4.6.3,
release content_shell build with enabled 'linux_dump_symbols' flag).

Results before:
avg 6637.456999820087 runs/s
median 6641.015455167277 runs/s
stdev 46.13222715581931 runs/s
min 6569.627789536735 runs/s
max 6713.072528670207 runs/s

Results after:
avg 6882.477918213916 runs/s
median 6869.921240991091 runs/s
stdev 43.08440888793655 runs/s
min 6823.600595470407 runs/s
max 6976.718834587643 runs/s

The average runs count increase is approx 4%.

Review URL: https://codereview.chromium.org/225393003

git-svn-id: svn://svn.chromium.org/blink/trunk@170891 bbb929c8-8fbe-4397-9dbb-9b2b20218538