Move strings and collection class buffers into the WTF buffer partition.

This patch also removes two static initializers / destructors.
Blink coding style is to generally avoid these by having a static pointer to the
non-POD type, initializing it with an inline if, and letting it leak. However,
there are a few instances where that style is violated and these are causing
trouble by illegally touching Blink objects after WebKit::shutdown has been
called.

Easy to fix: two sites switched to standard style.

BUG=270531

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159726

R=eseidel@chromium.org, inferno@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/25644002

git-svn-id: svn://svn.chromium.org/blink/trunk@160150 bbb929c8-8fbe-4397-9dbb-9b2b20218538