Fix Windows Debug build

This CL re-works StaticStrings to be allocated more conventionally, hopefully
healing the Windows bots. I'll investigate reducing the number of mallocs in a
later CL. For now, I just want my StringImpl-shrink CL to stick so that we can
make progress.

TBR=eseidel

git-svn-id: svn://svn.chromium.org/blink/trunk@155468 bbb929c8-8fbe-4397-9dbb-9b2b20218538