Stop inlining StringBuffer<CharType>::shrink().

Minor cleanup as part of looking at overly aggressive inlining. The default
for all code should be that no code is force-inlined unless it is
critical for some other reason.

This is a prerequisite for http://codereview.chromium.org/390003003/

Review URL: https://codereview.chromium.org/520723002

git-svn-id: svn://svn.chromium.org/blink/trunk@181193 bbb929c8-8fbe-4397-9dbb-9b2b20218538