Make MutableStylePropertySet::removePropertiesInSet faster

MutableStylePropertySet::removePropertiesInSet is used by several methods
removing CSS properties and it is pointed out as a hotspot by Intel VTune
Amplifier profile, when running 'CSSPropertySetterGetterMethods.html'
performance test

This patch makes the method faster by
- Avoiding an unneeded HashSet creation
- not using 'Vector::at' calls in cycle (avoid unneeded RELEASE_ASSERT)
- use 'Vector::swap' instead of assignment operator

Results are the following (linux desktop x64, GCC 4.6.3, release content_shell)

Before:
avg 6954.44302202544 runs/s
median 6966.681499614755 runs/s
stdev 40.17190255456056 runs/s
min 6868.852615417476 runs/s
max 7014.006093452284 runs/s

After:
avg 7431.277303542255 runs/s
median 7442.596071655415 runs/s
stdev 40.61578938819914 runs/s
min 7351.983886907447 runs/s
max 7497.940019131489 runs/s

Average runs count increase is approx 7%.

Review URL: https://codereview.chromium.org/227663002

git-svn-id: svn://svn.chromium.org/blink/trunk@171246 bbb929c8-8fbe-4397-9dbb-9b2b20218538