Remove a pointer from StringImpl

The m_buffer field is no longer needed. On 64 bit systems, this CL reduces the
sizeof(StringImpl) from 32 to 24 bytes.

The CL is a strict win, but I was curious how big an impact it has on actual
web sites so I looked at the STRING_STATS for the HTML Standard and ESPN.com.

== HTML Standard ==
Character data:                 798 kB (12065 strings)
StringImpl overhead (before):   386 kB (48% overhead)
StringImpl overhead (after):    290 kB (36% overhead)

== ESPN.com ==
Character data:               1,403 kB (10388 strings)
StringImpl overhead (before):   332 kB (24% overhead)
StringImpl overhead (after):    249 kB (18% overhead)

On these sites, StringImpl is a significant amount of overhead compared to the
total amount of character data actually stored in the strings. This CL saves
about 90 kB of memory and makes a noticeable dent in the total amount of memory
used by strings. Even after this CL, we're still spending a sizable amount of
memory on StringImpl overhead.

R=eseidel
BUG=262320

Review URL: https://chromiumcodereview.appspot.com/19654004

git-svn-id: svn://svn.chromium.org/blink/trunk@154641 bbb929c8-8fbe-4397-9dbb-9b2b20218538