Explicitly inline hot return value setter funcs in UnsafePersistent.

It doesn't make a perf difference on Linux, but, as we use explicit inline
directives on hot paths, it makes sense to explicitly inline these too.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23685002

git-svn-id: svn://svn.chromium.org/blink/trunk@156840 bbb929c8-8fbe-4397-9dbb-9b2b20218538