Simplify & optimize StringImpl::replace(UChar, UChar)
Simplify & optimize StringImpl::replace(UChar, UChar)

StringImpl::replace(UChar, UChar) is mentioned as one of the
hot spots by the Intel VTune Amplifier profile when running
the 'textarea-dom.html' performance test.

StringImpl::replace(UChar, UChar) is optimized (and simplified)
by using 'StringImpl::find' method instead of manual traversing
the internal buffer and checking 'is8Bit' property value on
each iteration.

This changes gives approx 2.25% improvement of the average
'textarea-dom.html' performance test results (linux desktop x64,
GCC 4.6.3, release content_shell build with enabled
'linux_dump_symbols' flag).

Review URL: https://codereview.chromium.org/304273003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@175198 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs