Remove StringImpl::m_substringBuffer

There aren't enough clients of this pointer to be worth keeping the feature in
StringImpl anymore. This CL paves the way to shrinking every StringImpl by one
pointer.

R=eseidel@chromium.org
BUG=262320

Review URL: https://codereview.chromium.org/19749009

git-svn-id: svn://svn.chromium.org/blink/trunk@154628 bbb929c8-8fbe-4397-9dbb-9b2b20218538